ext/ffmpeg/: Do proper locking around avcodec_open() and avcodec_close()

ext/ffmpeg/: Do proper locking around avcodec_open() and avcodec_close()

Original commit message from CVS:
* ext/ffmpeg/gstffmpeg.c: (gst_ffmpeg_avcodec_open),
(gst_ffmpeg_avcodec_close):
* ext/ffmpeg/gstffmpeg.h:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_close),
(gst_ffmpegdec_open):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_dispose),
(gst_ffmpegenc_getcaps), (gst_ffmpegenc_setcaps),
(gst_ffmpegenc_change_state):
Do proper locking around avcodec_open() and avcodec_close()
(fixes #322254, patch by: Sebastien Cote and Luca Ognibene).