adaptivedemux: deadlock on task stop

Submitted by Ravi

Created attachment 362871
if the streams are not cancelled during task stop and we are in prerolling then unblock the data push thread.

The deadlock is observed when we quickly do start-stop pipeline.

The other use case if you push new mpd buffer to adaptivedemux and then shutdown the pipeline.

Patch 362871, "if the streams are not cancelled during task stop and we are in prerolling then unblock the data push thread.":
deadlockFix.patch

Version: 1.12.0