explicitly shutdown engine at exit

Callbacks like EngineHalted or DeviceListChanged 
can be deadly while the UI is being torn down.

gtk2_ardour/main.cc

@@ -301,6 +301,7 @@ int main (int argc, char *argv[])
 	}
 
 	ui->run (text_receiver);
+	ui->disconnect_from_engine();
 	Gtkmm2ext::Application::instance()->cleanup();
 	delete ui;
 	ui = 0;