matroskademux: Guard event deferring with locks

When handling seek events, the demuxer object lock must be taken before
checking the read state and storing the event in order to avoid
potential races in case the streaming thread is in the middle of a read
state transition.
Status Job ID Name Coverage
  Manifest
passed #46633
manifest

00:00:23

 
  Build
passed #46635
build fedora x86_64

00:01:26

failed #46634
build fedora x86_64

00:02:13