Don't segfault if an empty codebook gets used.

      Patch by Denes Balatoni: [ dbalatoni interware hu ]
Original thread: [Ffmpeg-devel] [PATCH/BUGREPORT] crash in vorbis decoder
date: 02/04/2007 11:08 PM

Originally committed as revision 9711 to svn://svn.ffmpeg.org/ffmpeg/trunk