Backport of "rtspclientsink: fix deadlock on shutdown if no data has been received yet" into 1.18
requested to merge tpm/gst-rtsp-server:backport-mr-189-into-1.18-rtspclientsink-shutdown-before-any-data-received into 1.18
Backport of !189 (merged) "rtspclientsink: fix deadlock on shutdown if no data has been received yet".
Commits: