cc708overlay: fix deadlock

We would forget to unlock when a caption data buffer is deemed
out of segment, which makes everything lock up next time buffers
are received.
3 jobs for cc708overlay-deadlock in 10 minutes and 10 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #53017
manifest

00:00:28

 
  Build
passed #53018
build fedora x86_64

00:06:07

 
  Full Builds
passed #53019
build android arm64 api28

00:03:35