v4l2videodec: Check stop in flush() to avoid race condition.

Backward playback will drain and flush every frame. Stop playback
when backward playback have race condition between exit thread and
streaming thread flush. Add one check to avoid it.

Fixes #639
25 jobs for flush in 69 minutes and 16 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #715878
gst indent

00:00:37

passed #715877
manifest

00:00:49

 
  Build
passed #715888
build cerbero cross-android universal

00:16:47

manual #715892
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #715889
build cerbero cross win32

00:08:13

passed #715890
build cerbero cross win64

00:06:28

passed #715887
build cerbero fedora x86_64

00:07:17

passed #715891
gst-macos-10.14
build cerbero macos x86_64

00:06:59

passed #715883
build clang fedora x86_64

00:08:13

passed #715879
build fedora x86_64

00:06:04

passed #715886
docker windows 1607
build msys2

00:11:53

passed #715880
build nodebug fedora x86_64

00:05:29

passed #715881
build static fedora x86_64

00:13:53

passed #715882
build static nodebug fedora x86_64

00:04:58

passed #715884
docker windows 1607
build vs2017 amd64

00:11:12

passed #715885
docker windows 1607
build vs2017 x86

00:10:05

 
  Test
passed #715893
check fedora

00:06:50

passed #718284
integration testsuites fedora

00:40:35

passed #715896
valgrind bad

00:16:35

passed #715897
valgrind ges

00:06:43

passed #715895
valgrind good

00:09:13

failed #715894
integration testsuites fedora

01:00:00

 
  Integrate
passed #715898
cross-android universal examples

00:07:56

manual #715900
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #715899
documentation

00:04:23