InstrumentTrack: check for instrument instance in processOutEvent()

There are situations where processOutEvent() is being called while
loading instrument track settings e.g. when loading the pitch model
whose dataChanged() signal is connected to InstrumentTrack::updatePitch()
which in turn calls processOutEvent(). At this time we do not have an
instrument instance and therefore have to skip processing.

Closes #164.

src/tracks/InstrumentTrack.cpp

@@ -356,6 +356,12 @@ void InstrumentTrack::processInEvent( const MidiEvent& event, const MidiTime& ti
 
 void InstrumentTrack::processOutEvent( const MidiEvent& event, const MidiTime& time )
 {
+	// do nothing if we do not have an instrument instance (e.g. when loading settings)
+	if( m_instrument == NULL )
+	{
+		return;
+	}
+
 	const MidiEvent transposedEvent = applyMasterKey( event );
 	const int key = transposedEvent.key();