InstrumentTrack: fix crash in destructor with IPH-based instruments

Commit e3fa8762ef introduced a crash
in destructor of InstrumentTrack when using IPH based instruments.
Therefore explicitely remove all play handles after calling
silenceAllNotes().

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

src/tracks/instrument_track.cpp

@@ -139,6 +139,8 @@ instrumentTrack::~instrumentTrack()
 	// kill all running notes
 	silenceAllNotes();
 
+	engine::getMixer()->removePlayHandles( this );
+
 	// now we're save deleting the instrument
 	delete m_instrument;
 }