ext/ffmpeg/: Protect calls to ffmpeg functions that will implicitly call avcodec_open/close with the avcodec mutex.

Original commit message from CVS:
* ext/ffmpeg/gstffmpeg.c: (gst_ffmpeg_av_find_stream_info):
* ext/ffmpeg/gstffmpeg.h:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_open):
Protect calls to ffmpeg functions that will implicitly call
avcodec_open/close with the avcodec mutex.
Fixes #433245