avcodec/diracdec: Check numx/y

Fixes division by 0
Fixes: 60261c4469ba3e11059890fb2832a515/asan_generic_135e694_2790_beb94eaa0aeb7d11c0437375a8964a99.drc

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
