fuzz-unit-file: add __has_feature(memory_sanitizer) when skipping ListenNetlink=

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.

src/basic/cgroup-util.c

@@ -1976,7 +1976,7 @@ int cg_slice_to_path(const char *unit, char **ret) {
                 _cleanup_free_ char *escaped = NULL;
                 char n[dash - p + sizeof(".slice")];
 
-#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+#if HAS_FEATURE_MEMORY_SANITIZER
                 /* msan doesn't instrument stpncpy, so it thinks
                  * n is later used unitialized:
                  * https://github.com/google/sanitizers/issues/926