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).


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@17945 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b