widget: Take a ref during ::unmap emission

This ensures the widget data stays live long enough to perform invariants
check after emission if the widget happens to be destroyed on a callback.

https://bugzilla.gnome.org/show_bug.cgi?id=745829

gtk/gtkwidget.c

@@ -5072,6 +5072,7 @@ gtk_widget_unmap (GtkWidget *widget)
 
   if (gtk_widget_get_mapped (widget))
     {
+      g_object_ref (widget);
       gtk_widget_push_verify_invariants (widget);
 
       if (!gtk_widget_get_has_window (widget))
@@ -5084,6 +5085,7 @@ gtk_widget_unmap (GtkWidget *widget)
       g_signal_emit (widget, widget_signals[UNMAP], 0);
 
       gtk_widget_pop_verify_invariants (widget);
+      g_object_unref (widget);
     }
 }