avcodec/zmbv: Check len before reading in decode_frame()

Fixes out of array read
Fixes: asan_heap-oob_4d4eb0_3994_cov_3169972261_zmbv_15bit.avi

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