avcodec/pngdec: Check bits per pixel before setting monoblack pixel format

      Fixes out of array accesses
Fixes: asan_heap-oob_14dbfcf_4_asan_heap-oob_1ce5767_179_add_method_small.png

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