Assign fctx->client when fctx is created rather when the join happens.

This prevents races on fctx->client whenever a new fetch joins a existing
fetch (by calling fctx_join) as it is now invariant for the active life of
fctx.

(cherry picked from commit 9ca6ad63)
65 jobs for !2694 with 1416-threadsanitizer-data-race-resolver-c-3384-in-findname-v9_14 in 20 minutes and 18 seconds (queued for 9 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
passed #448427
amd64 linux
🐞:sid:amd64

00:00:52

passed #448425
amd64 linux
autoreconf:sid:amd64

00:00:18

passed #448426
amd64 linux
misc:sid:amd64

00:00:19

 
  Build
passed #448440
amd64 linux
asan:sid:amd64

00:02:01

passed #448444
amd64 freebsd
clang:freebsd11.3:amd64

00:01:22

passed #448445
amd64 freebsd
clang:freebsd12.0:amd64

00:01:10

passed #448446
amd64 openbsd
clang:openbsd6.5:amd64

00:04:30

passed #448441
amd64 linux
clang:stretch:amd64

00:01:39

passed #448442
i386 linux
clang:stretch:i386

00:01:39

passed #448428
amd64 linux
gcc:alpine3.10:amd64

00:02:05

passed #448439
amd64 linux
gcc:bionic:amd64

00:01:27

passed #448434
amd64 linux
gcc:buster:amd64

00:01:40

passed #448429
amd64 linux
gcc:centos6:amd64

00:02:02

passed #448430
amd64 linux
gcc:centos7:amd64

00:02:13

passed #448431
amd64 linux
gcc:centos8:amd64

00:02:24

passed #448437
amd64 linux
gcc:fedora30:amd64

00:02:27

passed #448432
amd64 linux
gcc:jessie:amd64

00:02:13

passed #448435
amd64 linux
gcc:sid:amd64

00:03:48

passed #448436
i386 linux
gcc:sid:i386

00:02:54

passed #448433
amd64 linux
gcc:stretch:amd64

00:01:30

passed #448438
amd64 linux
gcc:xenial:amd64

00:01:23

passed #448448
amd64 windows
msvc:windows:amd64

00:03:46

passed #448447
amd64 linux
nolibtool:sid:amd64

00:01:40

passed #448443
amd64 linux
pkcs11:sid:amd64

00:00:58

 
  Unit
passed #448461
amd64 linux
unit:asan:sid:amd64

00:00:57

passed #448464
amd64 freebsd
unit:clang:freebsd11.3:amd64

00:00:21

passed #448465
amd64 freebsd
unit:clang:freebsd12.0:amd64

00:00:22

passed #448462
amd64 linux
unit:clang:stretch:amd64

00:00:44

passed #448449
amd64 linux
unit:gcc:alpine3.10:amd64

00:00:44

passed #448460
amd64 linux
unit:gcc:bionic:amd64

00:00:44

passed #448455
amd64 linux
unit:gcc:buster:amd64

00:00:43

passed #448450
amd64 linux
unit:gcc:centos6:amd64

00:00:46

passed #448451
amd64 linux
unit:gcc:centos7:amd64

00:00:45

passed #448452
amd64 linux
unit:gcc:centos8:amd64

00:00:31

passed #448458
amd64 linux
unit:gcc:fedora30:amd64

00:00:48

passed #448453
amd64 linux
unit:gcc:jessie:amd64

00:00:28

passed #448456
amd64 linux
unit:gcc:sid:amd64

00:00:43

passed #448457
i386 linux
unit:gcc:sid:i386

00:00:42

passed #448454
amd64 linux
unit:gcc:stretch:amd64

00:00:43

passed #448459
amd64 linux
unit:gcc:xenial:amd64

00:00:32

passed #448466
amd64 linux
unit:nolibtool:sid:amd64

00:01:02

passed #448463
amd64 linux
unit:pkcs11:sid:amd64

00:00:42

 
  System
passed #448479
amd64 linux
system:asan:sid:amd64

00:08:50

passed #448481
amd64 freebsd
system:clang:freebsd11.3:amd64

00:06:27

passed #448482
amd64 freebsd
system:clang:freebsd12.0:amd64

00:06:37

passed #448467
amd64 linux
system:gcc:alpine3.10:amd64

00:07:08

passed #448478
amd64 linux
system:gcc:bionic:amd64

00:06:41

passed #448473
amd64 linux
system:gcc:buster:amd64

00:06:42

passed #448468
amd64 linux
system:gcc:centos6:amd64

00:06:58

passed #448469
amd64 linux
system:gcc:centos7:amd64

00:06:48

passed #448557
linux amd64
system:gcc:centos8:amd64

00:06:43

passed #448558
linux amd64
system:gcc:fedora30:amd64

00:06:34

passed #448556
linux amd64
system:gcc:jessie:amd64

00:06:30

passed #448474
amd64 linux
system:gcc:sid:amd64

00:07:23

passed #448475
i386 linux
system:gcc:sid:i386

00:08:03

passed #448472
amd64 linux
system:gcc:stretch:amd64

00:07:55

passed #448477
amd64 linux
system:gcc:xenial:amd64

00:06:30

passed #448483
amd64 linux
system:nolibtool:sid:amd64

00:06:58

passed #448480
amd64 linux
system:pkcs11:sid:amd64

00:06:40

failed #448470
amd64 linux
system:gcc:centos8:amd64

00:08:25

failed #448476
amd64 linux
system:gcc:fedora30:amd64

00:08:07

failed #448471
amd64 linux
system:gcc:jessie:amd64

00:06:48

 
  Docs
passed #448484
amd64 linux
docs:sid:amd64

00:00:57

 
  Postcheck
passed #448486
amd64 linux
cppcheck:gcc:sid:amd64

00:04:16

passed #448485
amd64 linux
scan-build:buster:amd64

00:10:41