avcodec/smc: fix off by 1 error

Fixes out of array access
Fixes: asan_heap-oob_1685bf0_5_asan_heap-oob_1f35116_430_smc.mov

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