avcodec/avrndec: Use the AVFrame format instead of the context

Fixes out of array read
Fixes: 20dd01398dee0f6d83d7e5410a2ae8eb/signal_sigsegv_39eeb1f_4001_62efbdf1c60748dabf1ec310b59525fd.mov

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