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.
26 jobs for failed-msdk-context in 30 minutes and 30 seconds
latest
Status Job ID Name Coverage
  Preparation
passed #2033428
gst indent

00:00:30

passed #2033427
manual
manifest

00:00:23

 
  Build
passed #2033438
gstreamer
build cerbero cross-android universal

00:11:55

passed #2033442
gst-ios-13.2
build cerbero cross-ios universal

00:19:53

passed #2033439
gstreamer
build cerbero cross win32

00:08:28

passed #2033440
gstreamer
build cerbero cross win64

00:03:57

passed #2033437
gstreamer
build cerbero fedora x86_64

00:05:50

passed #2033441
gst-macos-10.15
build cerbero macos x86_64

00:12:12

passed #2033433
build clang fedora x86_64

00:08:58

passed #2033429
build fedora x86_64

00:07:01

manual #2033436
docker windows 1809 allowed to fail manual
build msys2
passed #2033430
build nodebug fedora x86_64

00:05:03

passed #2033431
build static fedora x86_64

00:14:26

passed #2033432
build static nodebug fedora x86_64

00:05:03

passed #2033434
docker windows 1809
build vs2017 amd64

00:11:59

passed #2033435
docker windows 1809
build vs2017 x86

00:12:23

 
  Test
passed #2033896
gstreamer
check fedora

00:03:46

passed #2033444
gstreamer
integration testsuites fedora 1/4

00:14:46

passed #2033445
gstreamer
integration testsuites fedora 2/4

00:20:12

passed #2033446
gstreamer
integration testsuites fedora 3/4

00:13:33

passed #2033447
gstreamer
integration testsuites fedora 4/4

00:14:02

passed #2033448
gstreamer
valgrind bad

00:07:18

failed #2033443
gstreamer
check fedora

00:04:08

 
  Integrate
passed #2033449
cross-android universal examples

00:06:38

passed #2033451
gst-ios-13.2
cross-ios universal examples

00:02:49

passed #2033450
documentation

00:04:21