avformat/ipmovie: Check that the OPCODE_INIT_AUDIO_BUFFERS size is large enough

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f75b03c1f19_4820_descent3_level5_16bit_partial.mve
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
