avcodec/hnm4video: check offset before subtraction in decode_interframe_v4a()

Fixes out of array read
Fixes: signal_sigsegv_1326a09_1752_cov_245452111_GRTH301.HNS
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>