libs: dec: vp9: clear parser pointer after release

Fix an use-after-release of the parser pointer in VP9 decoder.
6 jobs for vp9-fix-use-after-release in 17 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #328603
gst indent

00:00:23

passed #328602
manifest

00:00:43

 
  Build
passed #328604
build fedora x86_64

00:08:59

passed #328605
build nodebug fedora x86_64

00:05:00

 
  Test
passed #328606
check fedora

00:04:27

 
  Deploy
passed #328607
documentation

00:03:24