stack: protect set_visible_child_name from NULL stack

Return with error if gtk_stack_set_visible_child_name is called
with NULL parameter

gtk/gtkstack.c

@@ -1836,6 +1836,8 @@ gtk_stack_set_visible_child_name (GtkStack   *stack,
 {
   GtkStackPrivate *priv = gtk_stack_get_instance_private (stack);
 
+  g_return_if_fail (GTK_IS_STACK (stack));
+
   gtk_stack_set_visible_child_full (stack, name, priv->transition_type);
 }