alsdec: Fix the clipping range

mcc_weightings is only 32 elements.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
alsdec: Clean up error paths

Fix at least a memory leak.

CC: libav-stable@libav.org
4xm: Reject not a multiple of 16 dimension

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
8bps: Bound-check the input buffer

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
8bps: Make the bound-checks consistent