avcodec/dnxhddec: Make mb_scan_index a fixed length array

Fixes null pointer dereference
Fixes: 5c9d1a6f74a12763fc7c9dd7834022b9/signal_sigsegv_11f78d9_1461_ecee3c5e7205457498e79b3ffaf21d0c.mxf

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