listbox: unparent new header only if it has a parent

It's no longer OK to unparent a widget that has no parent.

gtk/gtklistbox.c

@@ -2232,7 +2232,8 @@ gtk_list_box_update_header (GtkListBox    *box,
           if (new_header != NULL)
             {
               g_hash_table_insert (box->header_hash, new_header, row);
-              gtk_widget_unparent (new_header);
+              if (gtk_widget_get_parent (new_header) != NULL)
+                gtk_widget_unparent (new_header);
               gtk_widget_set_parent (new_header, GTK_WIDGET (box));
               gtk_widget_show (new_header);
             }

gtk/gtkwidget.c

@@ -2497,7 +2497,11 @@ gtk_widget_unparent (GtkWidget *widget)
   g_return_if_fail (GTK_IS_WIDGET (widget));
 
   if (priv->parent == NULL)
-    return;
+    {
+      g_warning ("Attempted to unparent %s %p, but it already has no parent.",
+                 G_OBJECT_TYPE_NAME (widget), widget);
+      return;
+    }
 
   gtk_widget_push_verify_invariants (widget);