avformat/ipmovie: Fix use of uninitialized memory in OPCODE_INIT_VIDEO_BUFFERS

Fixes: msan_uninit-mem_7ffe323a25f3_5929_ipmovie_interplayvideo_interplay_dpcm__bislogo.mve
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
