systemctl: unset const char* arguments in static destructors

When fuzzing, the following happens:
- we parse 'data' and produce an argv array,
- one of the items in argv is assigned to arg_host,
- the argv array is subsequently freed by strv_freep(), and arg_host has a dangling symlink.

In normal use, argv is static, so arg_host can never become a dangling pointer.
In fuzz-systemctl-parse-argv, if we repeatedly parse the same array, we
have some dangling pointers while we're in the middle of parsing. If we parse
the same array a second time, at the end all the dangling pointers will have been
replaced again. But for a short time, if parsing one of the arguments uses another
argument, we would use a dangling pointer.

Such a case occurs when we have --host=… --boot-loader-entry=help. The latter calls
acquire_bus() which uses arg_host.

I'm not particularly happy with making the code more complicated just for
fuzzing, but I think it's better to resolve this, even if the issue cannot
occur in normal invocations, than to deal with fuzzer reports.

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=31714.

src/basic/alloc-util.h

@@ -79,6 +79,13 @@ void* memdup_suffix0(const void *p, size_t l); /* We can't use _alloc_() here, s
                 memcpy_safe(_q_, p, _l_);       \
         })
 
+static inline void unsetp(void *p) {
+        /* A trivial "destructor" that can be used in cases where we want to
+         * unset a pointer from a _cleanup_ function. */
+
+        *(void**)p = NULL;
+}
+
 static inline void freep(void *p) {
         *(void**)p = mfree(*(void**) p);
 }