heap-use-after-free caused by checking for duplicate "http" configurations

Checking for duplicate http clauses in configuration files leads to heap use after free.

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==1833==ERROR: AddressSanitizer: heap-use-after-free on address 0x60300002b420 at pc 0x7fbcc0f4c4f2 bp 0x7ffdd9e9a170 sp 0x7ffdd9e99920
READ of size 1 at 0x60300002b420 thread T0
    #0 0x7fbcc0f4c4f1  (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb64f1)
    #1 0x7fbcc0b2dacd in isc_symtab_define /builds/isc-projects/bind9/lib/isc/symtab.c:221
    #2 0x7fbcbe556dfc in bind9_check_httpserver /builds/isc-projects/bind9/lib/bind9/check.c:2046
    #3 0x7fbcbe556dfc in bind9_check_httpservers /builds/isc-projects/bind9/lib/bind9/check.c:2111
    #4 0x7fbcbe556dfc in bind9_check_namedconf /builds/isc-projects/bind9/lib/bind9/check.c:5692
    #5 0x55798af6ceb7 in main /builds/isc-projects/bind9/bin/check/named-checkconf.c:726
    #6 0x7fbcbd83e09a in __libc_start_main ../csu/libc-start.c:308
    #7 0x55798af697c9 in _start (/builds/isc-projects/bind9/bin/check/.libs/named-checkconf+0xa7c9)

0x60300002b420 is located 0 bytes inside of 18-byte region [0x60300002b420,0x60300002b432)
freed by thread T0 here:
    #0 0x7fbcc0f7efb0 in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xe8fb0)
    #1 0x7fbcc0ac2ca6 in sdallocx /builds/isc-projects/bind9/lib/isc/jemalloc_shim.h:39
    #2 0x7fbcc0ac2ca6 in mem_put /builds/isc-projects/bind9/lib/isc/mem.c:361
    #3 0x7fbcc0ac2ca6 in isc__mem_free /builds/isc-projects/bind9/lib/isc/mem.c:977
    #4 0x7fbcbe556e22 in bind9_check_httpserver /builds/isc-projects/bind9/lib/bind9/check.c:2066
    #5 0x7fbcbe556e22 in bind9_check_httpservers /builds/isc-projects/bind9/lib/bind9/check.c:2111
    #6 0x7fbcbe556e22 in bind9_check_namedconf /builds/isc-projects/bind9/lib/bind9/check.c:5692
    #7 0x55798af6ceb7 in main /builds/isc-projects/bind9/bin/check/named-checkconf.c:726
    #8 0x7fbcbd83e09a in __libc_start_main ../csu/libc-start.c:308

previously allocated by thread T0 here:
    #0 0x7fbcc0f7f330 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xe9330)
    #1 0x7fbcc0ac1020 in mallocx /builds/isc-projects/bind9/lib/isc/jemalloc_shim.h:29
    #2 0x7fbcc0ac1020 in mem_get /builds/isc-projects/bind9/lib/isc/mem.c:341
    #3 0x7fbcc0ac1020 in isc__mem_allocate /builds/isc-projects/bind9/lib/isc/mem.c:886
    #4 0x7fbcc0ac429b in isc__mem_strdup /builds/isc-projects/bind9/lib/isc/mem.c:996
    #5 0x7fbcbe556d8b in bind9_check_httpserver /builds/isc-projects/bind9/lib/bind9/check.c:2039
    #6 0x7fbcbe556d8b in bind9_check_httpservers /builds/isc-projects/bind9/lib/bind9/check.c:2111
    #7 0x7fbcbe556d8b in bind9_check_namedconf /builds/isc-projects/bind9/lib/bind9/check.c:5692
    #8 0x55798af6ceb7 in main /builds/isc-projects/bind9/bin/check/named-checkconf.c:726
    #9 0x7fbcbd83e09a in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: heap-use-after-free (/usr/lib/x86_64-linux-gnu/libasan.so.5+0xb64f1) 
Shadow bytes around the buggy address:
  0x0c067fffd630: 00 00 00 fa fa fa 00 00 02 fa fa fa 00 00 00 fa
  0x0c067fffd640: fa fa 00 00 00 fa fa fa 00 00 00 00 fa fa 00 00
  0x0c067fffd650: 00 fa fa fa 00 00 00 fa fa fa 00 00 00 00 fa fa
  0x0c067fffd660: 00 00 02 fa fa fa 00 00 00 fa fa fa 00 00 00 fa
  0x0c067fffd670: fa fa 00 00 00 00 fa fa 00 00 02 fa fa fa 00 00
=>0x0c067fffd680: 00 fa fa fa[fd]fd fd fa fa fa 00 00 02 fa fa fa
  0x0c067fffd690: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fffd6a0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fffd6b0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fffd6c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fffd6d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==1833==ABORTING

The problem was found by accident while working on a similar code in !5444 (merged)