avformat/ipmovie: Check that OPCODE_SET_PALETTE size is large enough

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7fec1f40656c_4819_descent3_level5_16bit_partial.mve
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
