matroska: Initialise a video_context field to satisfy valgrind

Clear the mastering_display_info_present field explicitly
after reallocating the track context into a video context
to avoid uninitialised warnings in valgrind
19 jobs for matroska-valgrind-fix in 41 minutes and 55 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #361994
gst indent

00:00:28

passed #361993
manifest

00:00:41

 
  Build
passed #362000
build cerbero android universal

00:10:56

passed #362001
build cerbero cross win32

00:10:08

passed #362002
build cerbero cross win64

00:07:55

passed #361999
build cerbero fedora x86_64

00:08:05

passed #362003
gst-macos-10.14
build cerbero macos x86_64

00:08:23

passed #361995
build fedora x86_64

00:08:45

passed #361996
build nodebug fedora x86_64

00:07:21

passed #361997
build static fedora x86_64

00:15:16

passed #361998
build static nodebug fedora x86_64

00:12:34

 
  Test
passed #362244
check fedora

00:03:24

passed #362005
integration testsuites fedora

00:16:42

passed #362007
valgrind bad

00:10:27

passed #362008
valgrind ges

00:09:42

passed #362006
valgrind good

00:12:47

failed #362004
check fedora

00:05:23

 
  Integrate
passed #362009
android universal examples

00:05:51

passed #362010
documentation

00:04:02