avformat/ipmovie: Check OPCODE_CREATE_TIMER size

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f81e836ef8c_5930_ipmovie_interplayvideo_interplay_dpcm__bislogo.mve
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
