PlayHandle: Zero out buffer before processing

Fixes buffer noises when instruments don't write the whole buffer, such as
bitinvader. Related:
* #3884 (comment): https://github.com/LMMS/lmms/pull/3884#issuecomment-337170598
* #3883
# #3383

src/core/PlayHandle.cpp

@@ -54,6 +54,7 @@ void PlayHandle::doProcessing()
 	if( m_usesBuffer )
 	{
 		m_bufferReleased = false;
+		BufferManager::clear(m_playHandleBuffer, Engine::mixer()->framesPerPeriod());
 		play( buffer() );
 	}
 	else