Prevent infinite loop when buffer holds SEQ_END_CODE only. Fix for r9870

Patch by Stefan Lucke <stefan <at> lucke.in-berlin.de>
Subject: API breakage by r9870
Date: 2007-10-05 11:06:47 GMT
http://thread.gmane.org/gmane.comp.video.ffmpeg.devel/57467

Originally committed as revision 10720 to svn://svn.ffmpeg.org/ffmpeg/trunk