avcodec/dnxhddec: treat pix_fmt like width/height

Fixes out of array accesses
Fixes: asan_heap-oob_22c9a39_16_015.mxf
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
