ext/ffmpeg/gstffmpegdec.c: Flush buffers in flush-stop, not flush-start, since the task is not stopped yet in flush-s...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_sink_event):
Flush buffers in flush-stop, not flush-start, since the task is not
stopped yet in flush-stop, which may lead to odd crashes in random
places inside libavcodec.