avcodec/snow: split block clipping checks

Fixes out of array read
Fixes: d4476f68ca1c1c57afbc45806f581963-asan_heap-oob_2266b27_8607_cov_4044577381_snow_chroma_bug.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>