mss12: validate display dimensions

The code currently reads the coded dimensions from the extradata, but
expects the display dimensions to be set by the caller, and does not
check that they are compatible (i.e. that the displayed size is smaller
than the coded size).

Make sure that when the display dimensions are set, they are also valid.
Fixes possible invalid memory access.

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