Set video width/height and create audio stream in read_packet instead of

Set video width/height and create audio stream in read_packet instead of
pre-parsing the file in read_header.
This avoids some code duplication and seeking, and also avoids an IO error
for small video-only files (as created during e.g. the FATE encoder test).

Originally committed as revision 17945 to svn://svn.ffmpeg.org/ffmpeg/trunk