Fix warnings:

ffmpeg.c: In function ‘new_video_stream’:
ffmpeg.c:3701:18: warning: ‘codec_id’ may be used uninitialized in this function
ffmpeg.c: In function ‘new_audio_stream’:
ffmpeg.c:3848:18: warning: ‘codec_id’ may be used uninitialized in this function
ffmpeg.c: In function ‘new_subtitle_stream’:
ffmpeg.c:3926:18: warning: ‘codec_id’ may be used uninitialized in this function

Originally committed as revision 26313 to svn://svn.ffmpeg.org/ffmpeg/trunk