support decoding of the last mpeg "packet" even if no startcode is immedeatly afterwards (fixes bugs with mpeg in avi)
new behavior is only activated if CODEC_FLAG_NOT_TRUNCATED is set


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@909 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b