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
24 jobs for flush in 44 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #732821
gst indent

00:00:38

passed #732820
manifest

00:00:53

 
  Build
passed #732831
build cerbero cross-android universal

00:11:46

manual #732835
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #732832
build cerbero cross win32

00:08:38

passed #732833
build cerbero cross win64

00:06:02

passed #732830
build cerbero fedora x86_64

00:06:56

passed #732834
gst-macos-10.14
build cerbero macos x86_64

00:07:43

passed #732826
build clang fedora x86_64

00:08:09

passed #732822
build fedora x86_64

00:04:21

passed #732829
docker windows 1607
build msys2

00:12:21

passed #732823
build nodebug fedora x86_64

00:04:08

passed #732824
build static fedora x86_64

00:13:13

passed #732825
build static nodebug fedora x86_64

00:05:02

passed #732827
docker windows 1607
build vs2017 amd64

00:11:03

passed #732828
docker windows 1607
build vs2017 x86

00:11:11

 
  Test
passed #732836
check fedora

00:04:28

passed #732837
integration testsuites fedora

00:39:36

passed #732839
valgrind bad

00:10:33

passed #732840
valgrind ges

00:07:12

passed #732838
valgrind good

00:08:07

 
  Integrate
passed #732841
cross-android universal examples

00:06:29

manual #732843
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #732842
documentation

00:05:13