use-of-uninitialized-value in IsGrayImage

use-of-uninitialized-value in IsGrayImage: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). MemorySanitizer: use-of-uninitialized-value in function IsGrayImage in file graphicsmagick/magick/analyze.c