undefined-behavior in vc1 decode ac coeff

undefined-behavior in vc1 decode ac coeff: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). UndefinedBehaviorSanitizer: undefined-behavior in function vc1 decode ac coeff in file ffmpeg/libavcodec/vc1 block.c