https://clang.llvm.org/docs/MemorySanitizer.html#id5 documents this check as the way to detect MemorySanitizer at compilation time. We only need to skip the test if MemorySanitizer is used. Also, use this condition in cg_slice_to_path(). There, the code that is conditionalized is not harmful in any way (it's just unnecessary), so remove the FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION condition. Fixes #8482.