systemctl: use free_and_replace on global variable

In normal usage we cannot set it multiple times, but from a fuzzer we
may. Doing it this way is nicer anyway.

src/systemctl/systemctl-compat-shutdown.c

@@ -132,9 +132,10 @@ int shutdown_parse_argv(int argc, char *argv[]) {
                 wall = argv + optind + 1;
 
         if (wall) {
-                arg_wall = strv_copy(wall);
-                if (!arg_wall)
+                char **copy = strv_copy(wall);
+                if (!copy)
                         return log_oom();
+                strv_free_and_replace(arg_wall, copy);
         }
 
         optind = argc;

src/systemctl/systemctl.c

@@ -1159,8 +1159,8 @@ static int run(int argc, char *argv[]) {
         case ACTION_SUSPEND_THEN_HIBERNATE:
         case ACTION_EMERGENCY:
         case ACTION_DEFAULT:
-                /* systemctl verbs with no equivalent in the legacy commands.  These cannot appear in
-                 * arg_action.  Fall through. */
+                /* systemctl verbs with no equivalent in the legacy commands. These cannot appear in
+                 * arg_action. Fall through. */
 
         case _ACTION_INVALID:
         default: