systemctl-clean-or-freeze: do not wait for units with --no-block

If --no-block is specified, the waiter is not allocated. But
bus_wait_for_units_run() was called unconditionally, and it asserts that
its argument is not NULL.

Only wait for the units when the waiter is allocated, as verb_kill()
does.

src/systemctl/systemctl-clean-or-freeze.c

@@ -95,11 +95,13 @@ int verb_clean_or_freeze(int argc, char *argv[], uintptr_t _data, void *userdata
                 }
         }
 
-        r = bus_wait_for_units_run(w);
-        if (r < 0)
-                return log_error_errno(r, "Failed to wait for units: %m");
-        if (r == BUS_WAIT_FAILURE)
-                ret = EXIT_FAILURE;
+        if (w) {
+                r = bus_wait_for_units_run(w);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to wait for units: %m");
+                if (r == BUS_WAIT_FAILURE)
+                        ret = EXIT_FAILURE;
+        }
 
         return ret;
 }