ext/ffmpeg/gstffmpegdec.c: Fix some segfaults while seeking. When i have unreffed the buffer in _release_buffer i sho...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_release_buffer),
(gst_ffmpegdec_frame):
Fix some segfaults while seeking. When i have unreffed
the buffer in _release_buffer i should also set picture->opaque to
NULL otherwise sometimes it can be unreffed another time in
_frame causing an invalid read/write.