avcodec/lcldec: Check that dimensions are a multiple of the subsample factors

Other dimensions would not work correctly currently,
also ask for a sample for files that fail this check.

This fixes an integer overflow leading to out of array
accesses.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
