decklinkvideosink: Don't forget to unref clock after usage

And don't unref a clock that is potentially NULL.
6 jobs for decklinkvideosink-clock-leak in 31 minutes and 4 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Preparation
passed #71685
gst indent

00:01:05

passed #71684
manifest

00:00:41

 
  Build
passed #71686
build fedora x86_64

00:04:44

 
  Test
passed #71687
check fedora

00:15:20

 
  Full Builds
passed #71688
build android arm64 api28

00:05:25

passed #71689
build cerbero fedora x86_64

00:09:53