avformat/ipmovie: check OPCODE_INIT_VIDEO_BUFFERS size more completely

Fixes use of uninitialized data

Fixes: signal_sigsegv_1571228_5930_ipmovie_interplayvideo_interplay_dpcm__bislogo.mve

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