fix ffmpeg locking issue

fix ffmpeg locking issue
ffmpeg complained "insufficient thread locking around avcodec_open/close()".
This was caused by encavcodec.c calling avcodec_open at the same time as
stream.c called av_find_stream_info.  av_find_stream_info has a side effect
of calling avcodec_open, so we must lock around this call as well.

git-svn-id: svn://localhost/HandBrake/trunk@3563 b64f7644-9d1e-0410-96f1-a4d463321fa5