Fix gtk_widget_get_allocation()

- add it to the header
- add the symbol to gtk.symbols
- fix coding style and check for allocation != NULL

gtk/gtk.symbols

@@ -4956,6 +4956,7 @@ gtk_widget_error_bell
 gtk_widget_event
 gtk_widget_freeze_child_notify
 gtk_widget_get_accessible
+gtk_widget_get_allocation
 gtk_widget_get_ancestor
 gtk_widget_get_app_paintable
 gtk_widget_get_child_requisition

gtk/gtkwidget.c

@@ -10686,9 +10686,11 @@ gtk_widget_get_has_tooltip (GtkWidget *widget)
  * Since: 2.18
  */
 void
-gtk_widget_get_allocation (GtkWidget *widget, GtkAllocation *allocation)
+gtk_widget_get_allocation (GtkWidget     *widget,
+                           GtkAllocation *allocation)
 {
   g_return_if_fail (GTK_IS_WIDGET (widget));
+  g_return_if_fail (allocation != NULL);
 
   *allocation = widget->allocation;
 }

gtk/gtkwidget.h

@@ -599,6 +599,9 @@ void                  gtk_widget_set_child_visible      (GtkWidget    *widget,
 gboolean              gtk_widget_get_child_visible      (GtkWidget    *widget);
 GdkWindow*            gtk_widget_get_window             (GtkWidget    *widget);
 
+void                  gtk_widget_get_allocation         (GtkWidget     *widget,
+                                                         GtkAllocation *allocation);
+
 gboolean   gtk_widget_child_focus         (GtkWidget           *widget,
                                            GtkDirectionType     direction);
 gboolean   gtk_widget_keynav_failed       (GtkWidget           *widget,