Fix signed overflow in _book_maptype1_quantvals().

If b->dims is very large, vals will be 1 and acc1 will overflow a
long.
This causes us to read too many values for this codebook.

Signed-off-by: Tristan Matthews <tmatth@videolan.org>

(cherry-picked from vorbis commit e5b1378996dba3ea82fb35403cb1f0bbff19495c)

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19619

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/tremor