headerbar: Don't leak widgets

We were not properly freeing separators that we end up
not using.

https://bugzilla.gnome.org/show_bug.cgi?id=774066

gtk/gtkheaderbar.c

@@ -455,8 +455,10 @@ _gtk_header_bar_update_window_buttons (GtkHeaderBar *bar)
 
           if (n_children == 0)
             {
-              gtk_widget_destroy (box);
-              gtk_widget_destroy (separator);
+              g_object_ref_sink (box);
+              g_object_unref (box);
+              g_object_ref_sink (separator);
+              g_object_unref (separator);
               continue;
             }