msdk: Fix abort when drm device won't open

If the drm device node fails to open or vaapi use fails, the msdkcontext
closes the session but does not initialize to NULL.
So, when the context is finalized, the session is double freed and abort occurs.
25 jobs for !1136 with failed-msdk-context in 32 minutes and 53 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Preparation
passed #2033899
gst indent

00:00:30

passed #2033898
manifest

00:00:23

 
  Build
passed #2033909
gstreamer
build cerbero cross-android universal

00:12:07

passed #2033913
gst-ios-13.2
build cerbero cross-ios universal

00:29:04

passed #2033910
gstreamer
build cerbero cross win32

00:06:31

passed #2033911
gstreamer
build cerbero cross win64

00:03:07

passed #2033908
gstreamer
build cerbero fedora x86_64

00:03:40

passed #2033912
gst-macos-10.15
build cerbero macos x86_64

00:10:19

passed #2033904
build clang fedora x86_64

00:07:27

passed #2033900
build fedora x86_64

00:06:17

manual #2033907
docker windows 1809 allowed to fail manual
build msys2
passed #2033901
build nodebug fedora x86_64

00:04:37

passed #2033902
build static fedora x86_64

00:11:57

passed #2033903
build static nodebug fedora x86_64

00:03:27

passed #2033905
docker windows 1809
build vs2017 amd64

00:11:54

passed #2033906
docker windows 1809
build vs2017 x86

00:12:09

 
  Test
passed #2033914
gstreamer
check fedora

00:03:36

passed #2033915
gstreamer
integration testsuites fedora 1/4

00:14:36

passed #2033916
gstreamer
integration testsuites fedora 2/4

00:15:30

passed #2033917
gstreamer
integration testsuites fedora 3/4

00:13:26

passed #2033918
gstreamer
integration testsuites fedora 4/4

00:19:34

passed #2033919
gstreamer
valgrind bad

00:07:41

 
  Integrate
passed #2033920
cross-android universal examples

00:06:38

passed #2033922
gst-ios-13.2
cross-ios universal examples

00:03:19

passed #2033921
documentation

00:04:34