SHOW_UBITS() is only defined up to n_bits is 25, therefore forbid
values larger than this in get_vlc2() (max_bits). tokens[][] can be
used as an index in deltas[], which has a size of 64, so ensure the
values are smaller than that.

This prevents crashes on corrupt bitstreams.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org