ext/ffmpeg/gstffmpegdec.c: make _open and _close as functions that should be called with the object lock instead of t...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_close),
(gst_ffmpegdec_setcaps), (gst_ffmpegdec_change_state):
make _open and _close as functions that should be
called with the object lock instead of them taking a
lock themselves. This fixes a deadlock when
_open fails.