avcodec/on2avc: Check number of channels

Fixes out of array access
Fixes: asan_heap-oob_4da4f3_7_asan_heap-oob_4da4f3_173_Xmen_avc_500.vp6

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