Handle edge case (optional toolbar meter)

in derivative projects it can happen that the meter is never
packaged in the custom toolbar. In Ardour this can happen
for sessions without master bus.

gtk2_ardour/application_bar.cc

@@ -805,7 +805,10 @@ ApplicationBar::set_session (Session *s)
 		_transport_hbox.remove (_editor_meter_table);
 	}
 	if (_editor_meter) {
-		_editor_meter_table.remove(*_editor_meter);
+		Gtk::Container *parent = _editor_meter->get_parent();
+		if (parent) {
+			parent->remove(*_editor_meter);
+		}
 		delete _editor_meter;
 		_editor_meter = 0;
 	}