Backport of "avwait: Don't post messages with the mutex locked" into 1.18
requested to merge tpm/gst-plugins-bad:backport-mr-2063-into-1.18-avwait-message-with-mutex into 1.18
Backport of !2063 (merged) "avwait: Don't post messages with the mutex locked".
Commits:
- 80c1722c avwait: Don't post messages with the mutex locked