vaapidecode: properly return from decode loop on downstream errors.

vaapidecode: properly return from decode loop on downstream errors.

Fixes a hang/race on shutdown where _decode_loop() had already completed
its execution and _finish() was waiting on a GCond for decode_loop()
to complete.  Also fixes the possible race where _finish() is called
but _decode_loop() endlessly returns before signalling completion
iff the decoder instance returns GST_FLOW_OK.

Found with: ... ! vaapidecode ! {glimagesink,cluttersink}

https://bugzilla.gnome.org/show_bug.cgi?id=733897

[factored out GST_VIDEO_DECODER_STREAM_UNLOCK() call]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>