Fix for issue #829 ("ffvorbis outputs crackling sound, libvorbis plays fine")

This was off-by-one bug which affects floor0 encoded files. Tracked down
by valgrind (was reported as the use of uninitialized memory).

With this patch applied, tiny_psnr now reports the following when ffvorbis
is compared to libvorbis-1.0beta4 output for 'lsp-test.ogg' sample:
stddev:    0.76 PSNR: 98.69 bytes:  3498380/  3498452

Output from libvorbis-1.2 is substantially different:
stddev:  132.57 PSNR: 53.87 bytes:  3498424/  3498496

Originally committed as revision 18821 to svn://svn.ffmpeg.org/ffmpeg/trunk
