needed to decode was not an integer number of dims wide.  This caused
it to overflow the output vector as the termination condition was in
the outer loop of vorbis_book_decodev_set.

None of the various vorbis_book_decodeXXXX calls internally guard
against this case either, but in every other use the calling code does
properly guard (and avoids putting more checks in the tight inner
decode loop).

For floor0, move the checks into the inner loop as there's little
penalty for doing so.  Add commentary indicating where guarding is
done for each call variant.




git-svn-id: https://svn.xiph.org/trunk/Tremor@17546 0101bb08-14d6-0310-b084-bc0e0c8e3800