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 Name Job ID Coverage
  Preparation
passed gst indent #2033899

00:00:30

passed manifest #2033898

00:00:23

 
  Build
passed build cerbero cross-android universal #2033909
gstreamer

00:12:07

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

00:29:04

passed build cerbero cross win32 #2033910
gstreamer

00:06:31

passed build cerbero cross win64 #2033911
gstreamer

00:03:07

passed build cerbero fedora x86_64 #2033908
gstreamer

00:03:40

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

00:10:19

passed build clang fedora x86_64 #2033904

00:07:27

passed build fedora x86_64 #2033900

00:06:17

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

00:04:37

passed build static fedora x86_64 #2033902

00:11:57

passed build static nodebug fedora x86_64 #2033903

00:03:27

passed build vs2017 amd64 #2033905
docker windows 1809

00:11:54

passed build vs2017 x86 #2033906
docker windows 1809

00:12:09

 
  Test
passed check fedora #2033914
gstreamer

00:03:36

passed integration testsuites fedora 1/4 #2033915
gstreamer

00:14:36

passed integration testsuites fedora 2/4 #2033916
gstreamer

00:15:30

passed integration testsuites fedora 3/4 #2033917
gstreamer

00:13:26

passed integration testsuites fedora 4/4 #2033918
gstreamer

00:19:34

passed valgrind bad #2033919
gstreamer

00:07:41

 
  Integrate
passed cross-android universal examples #2033920

00:06:38

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

00:03:19

passed documentation #2033921

00:04:34