avcodec/hnm4video: check offset in decode_interframe_v4() more completely

Fixes out of array reads
Fixes: signal_sigsegv_e74c1e_1092_BROCIME.HNM
Fixes: signal_sigsegv_e74e85_2620_PLAQUE0.HNM

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