PortManager::silence() should not clear data in AsyncMIDIPorts

This data was generated asynchronously and is not required to be "silenced"
the way that session data would be. Compare also to the similar tests for
sync-related MIDI ports

libs/ardour/port_manager.cc

@@ -684,6 +684,9 @@ PortManager::silence (pframes_t nframes, Session *s)
 		if (s && i->second == s->ltc_output_port ()) {
 			continue;
 		}
+		if (boost::dynamic_pointer_cast<AsyncMIDIPort>(i->second)) {
+			continue;
+		}
 		if (i->second->sends_output()) {
 			i->second->get_buffer(nframes).silence(nframes);
 		}