do not try to write from JackMIDIPort::write() if we are disconnected from JACK. should fix erroneous behaviour when MMC tries to send stop messages after JACK disconnects

git-svn-id: svn://localhost/ardour2/branches/3.0@13594 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/midi++2/jack_midi_port.cc

@@ -214,6 +214,13 @@ JackMIDIPort::write (const byte * msg, size_t msglen, timestamp_t timestamp)
 {
 	int ret = 0;
 
+	if (!_jack_client || !_jack_port) {
+		/* poof ! make it just vanish into thin air, since we are no
+		   longer connected to JACK.
+		*/
+		return msglen;
+	}
+
 	if (!sends_output()) {
 		return ret;
 	}