assert() which catches one source of crashes on startup when Ardour starts JACK.

git-svn-id: svn://localhost/ardour2/branches/3.0@7926 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/midi++2/port.cc

@@ -173,6 +173,8 @@ Port::clock (timestamp_t timestamp)
 void
 Port::cycle_start (nframes_t nframes)
 {
+	assert (_jack_port);
+	
 	_currently_in_cycle = true;
 	_nframes_this_cycle = nframes;