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/fuzz/fuzz-unit-file.c

@@ -48,7 +48,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 
                 ll = l + strspn(l, WHITESPACE);
 
-                if (startswith(ll, "ListenNetlink=")) {
+                if (HAS_FEATURE_MEMORY_SANITIZER && startswith(ll, "ListenNetlink=")) {
                         /* ListenNetlink causes a false positive in msan,
                          * let's skip this for now. */
                         log_notice("Skipping test because ListenNetlink= is present");