use-of-uninitialized-value in jxl u32

use-of-uninitialized-value in jxl u32: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). MemorySanitizer: use-of-uninitialized-value in function jxl u32 in file ffmpeg/libavcodec/jpegxl parse.c