Populate the buffer size combo when connecting to jack that is already running.

This would crash (throw) if ardour was started with an invalid backend
(alsa with no devices avaliable) and then changec to an alredy running
jack. The invalid backend at the start would leave the buffer
size combo empty and switching to the running jack backend did not populate
buffer size list.

gtk2_ardour/engine_dialog.cc

@@ -1032,6 +1032,7 @@ EngineControl::backend_changed ()
 
 	if (!_have_control) {
 		// set settings from backend that we do have control over
+		set_buffersize_popdown_strings ();
 		set_active_text_if_present (buffer_size_combo, bufsize_as_string (backend->buffer_size()));
 	}