avcodec/hnm4video: check that packetsize is at least 8

Fixes integer overflow and assertion failure
Fixes: signal_sigabrt_7ffff7126425_1243_CASSE.HNM

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