avcodec/hnm4video: fix write offset checks in decode_interframe_v4()

Fixes: asan_heap-oob_e76a51_1244_CASSE.HNM

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