MenuShell: Typecheck the instance @ select_first()

since this is public API and therefore might get junk passed to it.

gtk/gtkmenushell.c

@@ -1462,10 +1462,14 @@ void
 gtk_menu_shell_select_first (GtkMenuShell *menu_shell,
                              gboolean      search_sensitive)
 {
-  GtkMenuShellPrivate *priv = menu_shell->priv;
+  GtkMenuShellPrivate *priv;
   GtkWidget *to_select = NULL;
   GList *tmp_list;
 
+  g_return_if_fail (GTK_IS_MENU_SHELL (menu_shell));
+
+  priv = menu_shell->priv;
+
   tmp_list = priv->children;
   while (tmp_list)
     {