shell-util: Use early-return in shell_util_get_week_start()

Cleanup shell_util_get_week_start() a little bit and use an early-return
instead of setting the week_start variable to 0.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1546>

src/shell-util.c

@@ -128,7 +128,7 @@ shell_util_get_week_start (void)
     {
       g_warning ("Whoever translated calendar:week_start:0 for GTK+ "
                  "did so wrongly.\n");
-      week_start = 0;
+      return 0;
     }
 #endif