update engine dialog UI state

fixes crash when jack is started externally
and the dialog is displayed later.
The current state e.g. avail buffer sizes is not
are queried from the backend, Apply,OK -> crash
because the dropdown is empty)

gtk2_ardour/engine_dialog.cc

@@ -304,6 +304,7 @@ void
 EngineControl::on_show ()
 {
 	ArdourDialog::on_show ();
+	device_changed ();
 	ok_button->grab_focus();
 }