avcodec/interplayvideo: Check that enough data is available for opcode_0xA

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f90e5be6b30_3965_baldursgate_camp.mve
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
