Fixed a memory management error in the new codebook code

Fixed a memory management error in the new codebook code
Removed final-stage infinite shift buffer; now a no-copy
 double buffer; removes another 70kB nominal from decode
 with slight speed improvement (~2%)

there's still an exact-position seek bug found by seeking_example to
track down.  The seek succeeds, but the position is off.

Monty

svn path=/trunk/vorbis/; revision=2975