vorbisdec: Fix floor1 decoding

An intermediate value in the floor 1 linear interpolation was
overflowing
resulting in obvious artifacts on some files.

e.g.
http://upload.wikimedia.org/wikipedia/commons/7/79/Big_Buck_Bunny_small.ogv

Prior to this fix 87 out of 128 64kbit/s mono files decoded with ffmpeg
have
lower PEAQ ODG values than the same files decoded with libvorbis. With
this
fix none of that set have significantly worse ODG values than libvorbis.

Fixes issue 2352

Patch by Gregory Maxwell <greg@xiph.org>

Originally committed as revision 25724 to svn://svn.ffmpeg.org/ffmpeg/trunk
