Fix 'Dereference of null pointer' from scan-build-10

These are mostly false positives, the clang-analyzer FAQ[1] specifies
why and how to fix it:

> The reason the analyzer often thinks that a pointer can be null is
> because the preceding code checked compared it against null. So if you
> are absolutely sure that it cannot be null, remove the preceding check
> and, preferably, add an assertion as well.

The 4 warnings reported are:

dnssec-cds.c:781:4: warning: Access to field 'base' results in a dereference of a null pointer (loaded from variable 'buf')
                        isc_buffer_availableregion(buf, &r);
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/builds/isc-projects/bind9/lib/isc/include/isc/buffer.h:996:36: note: expanded from macro 'isc_buffer_availableregion'
                                   ^
/builds/isc-projects/bind9/lib/isc/include/isc/buffer.h:821:16: note: expanded from macro 'ISC__BUFFER_AVAILABLEREGION'
                (_r)->base = isc_buffer_used(_b);              \
                             ^~~~~~~~~~~~~~~~~~~
/builds/isc-projects/bind9/lib/isc/include/isc/buffer.h:152:29: note: expanded from macro 'isc_buffer_used'
        ((void *)((unsigned char *)(b)->base + (b)->used)) /*d*/
                                   ^~~~~~~~~
1 warning generated.

--

byname_test.c:308:34: warning: Access to field 'fwdtable' results in a dereference of a null pointer (loaded from variable 'view')
                RUNTIME_CHECK(dns_fwdtable_add(view->fwdtable, dns_rootname,
                                               ^~~~~~~~~~~~~~
/builds/isc-projects/bind9/lib/isc/include/isc/util.h:318:52: note: expanded from macro 'RUNTIME_CHECK'
                                                   ^~~~
/builds/isc-projects/bind9/lib/isc/include/isc/error.h:50:21: note: expanded from macro 'ISC_ERROR_RUNTIMECHECK'
        ((void)(ISC_LIKELY(cond) ||  \
                           ^~~~
/builds/isc-projects/bind9/lib/isc/include/isc/likely.h:23:43: note: expanded from macro 'ISC_LIKELY'
                                            ^
1 warning generated.

--

./rndc.c:255:6: warning: Dereference of null pointer (loaded from variable 'host')
        if (*host == '/') {
            ^~~~~
1 warning generated.

--

./main.c:1254:9: warning: Access to field 'sctx' results in a dereference of a null pointer (loaded from variable 'named_g_server')
        sctx = named_g_server->sctx;
               ^~~~~~~~~~~~~~~~~~~~
1 warning generated.

References:
1. https://clang-analyzer.llvm.org/faq.html#null_pointer
73 jobs for !3285 with ondrej/scan-build-10-fixes in 24 minutes and 9 seconds (queued for 6 minutes and 34 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
passed #783030
amd64 linux
🐞:sid:amd64

00:04:10

failed #783029
amd64 linux
💾:sid:amd64

00:00:10

passed #783027
amd64 linux
autoreconf:sid:amd64

00:00:32

passed #783028
amd64 linux
misc:sid:amd64

00:00:19

 
  Build
passed #783045
amd64 linux
asan:sid:amd64

00:04:48

passed #783052
amd64 libvirt
clang:freebsd11.3:amd64

00:01:39

passed #783053
amd64 libvirt
clang:freebsd12.1:amd64

00:01:52

passed #783054
amd64 libvirt
clang:openbsd6.6:amd64

00:04:57

failed #783049
amd64 linux
clang:stretch:amd64

00:00:27

failed #783050
i386 linux
clang:stretch:i386

00:00:55

passed #783031
amd64 linux
gcc:alpine3.11:amd64

00:02:38

passed #783044
amd64 linux
gcc:bionic:amd64

00:01:47

passed #783036
amd64 linux
gcc:buster:amd64

00:01:38

passed #783032
amd64 linux
gcc:centos6:amd64

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passed #783033
amd64 linux
gcc:centos7:amd64

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passed #783034
amd64 linux
gcc:centos8:amd64

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passed #783042
amd64 linux
gcc:fedora31:amd64

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passed #783037
amd64 linux
gcc:sid:amd64

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passed #783039
arm64 linux
gcc:sid:arm64

00:07:38

passed #783040
i386 linux
gcc:sid:i386

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passed #783035
amd64 linux
gcc:stretch:amd64

00:01:45

passed #783041
amd64 linux
gcc:tumbleweed:amd64

00:02:38

passed #783043
amd64 linux
gcc:xenial:amd64

00:01:40

passed #783055
amd64 windows
msvc:windows:amd64

00:02:39

passed #783048
amd64 linux
mutexatomics:sid:amd64

00:02:52

passed #783038
amd64 linux
oot:sid:amd64

00:02:58

passed #783051
amd64 linux
pkcs11:sid:amd64

00:02:34

passed #783047
amd64 linux
rwlock:sid:amd64

00:02:32

failed #783046
amd64 linux
tsan:buster:amd64

00:00:27

 
  Unit
passed #783069
amd64 linux
unit:asan:sid:amd64

00:01:06

passed #783075
amd64 libvirt
unit:clang:freebsd11.3:amd64

00:01:04

failed #783076
amd64 libvirt
unit:clang:freebsd12.1:amd64

00:05:53

skipped #783073
amd64 linux
unit:clang:stretch:amd64
passed #783056
amd64 linux
unit:gcc:alpine3.11:amd64

00:00:47

passed #783068
amd64 linux
unit:gcc:bionic:amd64

00:00:40

passed #783061
amd64 linux
unit:gcc:buster:amd64

00:00:56

passed #783057
amd64 linux
unit:gcc:centos6:amd64

00:00:52

passed #783058
amd64 linux
unit:gcc:centos7:amd64

00:00:52

passed #783059
amd64 linux
unit:gcc:centos8:amd64

00:00:53

passed #783066
amd64 linux
unit:gcc:fedora31:amd64

00:00:54

passed #783062
amd64 linux
unit:gcc:sid:amd64

00:00:57

passed #783063
arm64 linux
unit:gcc:sid:arm64

00:01:14

passed #783064
i386 linux
unit:gcc:sid:i386

00:01:04

passed #783060
amd64 linux
unit:gcc:stretch:amd64

00:00:40

passed #783065
amd64 linux
unit:gcc:tumbleweed:amd64

00:00:52

passed #783067
amd64 linux
unit:gcc:xenial:amd64

00:00:55

passed #783072
amd64 linux
unit:mutexatomics:sid:amd64

00:00:55

failed #783074
amd64 linux
unit:pkcs11:sid:amd64

00:01:35

passed #783071
amd64 linux
unit:rwlock:sid:amd64

00:00:52

skipped #783070
amd64 linux allowed to fail
unit:tsan:buster:amd64
 
  System
passed #783090
amd64 linux
system:asan:sid:amd64

00:12:20

passed #783095
amd64 libvirt
system:clang:freebsd11.3:amd64

00:12:19

failed #783096
amd64 libvirt
system:clang:freebsd12.1:amd64

00:12:13

passed #783077
amd64 linux
system:gcc:alpine3.11:amd64

00:10:48

passed #783089
amd64 linux
system:gcc:bionic:amd64

00:09:33

passed #783082
amd64 linux
system:gcc:buster:amd64

00:09:35

passed #783078
amd64 linux
system:gcc:centos6:amd64

00:09:54

passed #783079
amd64 linux
system:gcc:centos7:amd64

00:10:43

passed #783080
amd64 linux
system:gcc:centos8:amd64

00:10:21

passed #783087
amd64 linux
system:gcc:fedora31:amd64

00:10:07

passed #783083
amd64 linux
system:gcc:sid:amd64

00:10:44

passed #783084
arm64 linux
system:gcc:sid:arm64

00:11:17

passed #783085
i386 linux
system:gcc:sid:i386

00:10:50

passed #783081
amd64 linux
system:gcc:stretch:amd64

00:09:30

passed #783086
amd64 linux
system:gcc:tumbleweed:amd64

00:10:04

passed #783088
amd64 linux
system:gcc:xenial:amd64

00:09:11

passed #783093
amd64 linux
system:mutexatomics:sid:amd64

00:10:02

failed #783094
amd64 linux
system:pkcs11:sid:amd64

00:08:00

passed #783092
amd64 linux
system:rwlock:sid:amd64

00:10:49

skipped #783091
amd64 linux allowed to fail
system:tsan:buster:amd64
 
  Docs
passed #783097
amd64 linux
docs:sid:amd64

00:00:59

 
  Postcheck
failed #783099
amd64 linux
cppcheck:gcc:sid:amd64

00:00:44

failed #783098
amd64 linux
scan-build:buster:amd64

00:00:26

 
Name Stage Failure
failed
system:clang:freebsd12.1:amd64 System
No job log
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system:pkcs11:sid:amd64 System
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failed
unit:clang:freebsd12.1:amd64 Unit
No job log
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unit:pkcs11:sid:amd64 Unit
No job log
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cppcheck:gcc:sid:amd64 Postcheck
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scan-build:buster:amd64 Postcheck
No job log
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clang:stretch:i386 Build
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clang:stretch:amd64 Build
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tsan:buster:amd64 Build
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💾:sid:amd64 Precheck
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