avcodec/cinepak: fix integer underflow

      Fixes out of array access
Fixes: asan_heap-oob_4da0ba_6_asan_heap-oob_4da0ba_241_cvid_crash.avi

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