avcodec/snowdec: Fix ref value check

Fixes integer overflow and out of array read.
Fixes: signal_sigsegv_24169e6_3445_cov_3778346427_snow_chroma_bug.avi

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