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
26 jobs for flush in 70 minutes and 56 seconds (queued for 14 seconds)
Status Job ID Name Coverage
  Preparation
passed #610082
gst indent

00:00:31

passed #610081
manifest

00:00:39

 
  Build
passed #610092
build cerbero cross-android universal

00:57:07

manual #610096
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #610093
build cerbero cross win32

00:55:25

passed #610094
build cerbero cross win64

00:54:46

passed #610091
build cerbero fedora x86_64

00:45:33

passed #610095
gst-macos-10.14
build cerbero macos x86_64

00:53:56

passed #610087
build clang fedora x86_64

00:07:46

passed #610083
build fedora x86_64

00:07:25

passed #610090
docker windows 1607
build msys2

00:11:20

passed #610084
build nodebug fedora x86_64

00:05:28

passed #610085
build static fedora x86_64

00:13:46

passed #610086
build static nodebug fedora x86_64

00:04:07

passed #610088
docker windows 1607
build vs2017 amd64

00:11:52

passed #610089
docker windows 1607
build vs2017 x86

00:10:23

 
  Test
passed #610097
check fedora

00:04:16

created #634898
integration testsuites fedora
passed #610100
valgrind bad

00:07:44

passed #610101
valgrind ges

00:09:07

passed #610099
valgrind good

00:09:58

failed #610105
integration testsuites fedora

00:01:04

failed #610098
integration testsuites fedora

00:00:53

 
  Integrate
passed #610102
cross-android universal examples

00:09:22

created #610104
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #610103
documentation

00:06:44