fix routing midi data on export:

MidiPort::cycle_end() was never called, hence after the 
first cycle all midi buffers were assumed to be 
“mixed down” already.

this fixes 
Midi-track 1 -[midi]-> Midi-track2 synth -[audio]-> out
on export.

libs/ardour/audioengine.cc

@@ -333,10 +333,12 @@ AudioEngine::process_callback (pframes_t nframes)
 	}
 
 	if (_freewheeling) {
+		PortManager::cycle_end (nframes);
 		return 0;
 	}
 
 	if (!_running) {
+		PortManager::cycle_end (nframes);
 		_processed_frames = next_processed_frames;
 		return 0;
 	}