avcodec/mpeg4videodec: Check available data before reading custom matrix

Fixes: out of array read
Fixes: 76c515fc3779d1b838667c61ea13ce92/asan_heap-oob_1fc0d07_8913_794a4629a264ebdb25b58d3a94ed1785.bit

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