MIDI thread RT priority reworked a bit on OSX.

macosx/coremidi/JackCoreMidiOutputPort.cpp

@@ -55,6 +55,7 @@ JackCoreMidiOutputPort::Execute()
     for (;;) {
         MIDIPacket *packet = MIDIPacketListInit(packet_list);
         assert(packet);
+        assert(thread_queue);
         if (! event) {
             event = thread_queue->DequeueEvent((long) 0);
         }