InstrumentTrack: fix freeze when forwarding MIDI events to instrument

In commit a3abcdb2e0 we introduced
multitimbral MIDI support. However the new code path causes the mixer
not being unlocked at exit and thus causes a freeze of LMMS.

Thanks to nuio for pointing out this issue and providing a patch.

Closes #532.

src/tracks/InstrumentTrack.cpp

@@ -2,7 +2,7 @@
  * InstrumentTrack.cpp - implementation of instrument-track-class
  *                        (window + data-structures)
  *
- * Copyright (c) 2004-2012 Tobias Doerffel <tobydox/at/users.sourceforge.net>
+ * Copyright (c) 2004-2013 Tobias Doerffel <tobydox/at/users.sourceforge.net>
  *
  * This file is part of Linux MultiMedia Studio - http://lmms.sourceforge.net
  *
@@ -230,6 +230,7 @@ void InstrumentTrack::processInEvent( const midiEvent & _me,
 			midiPort()->realOutputChannel() < 0 */ )
 	{
 		m_instrument->handleMidiEvent( _me, _time );
+		engine::getMixer()->unlock();
 		return;
 	}