Session-cleanup: clear route references from graph

resort_routes() is a no-op during session destruction,
however _current_route_graph can still hold references
to routes. A GraphVertex is-a boost::shared_ptr<Route>.

libs/ardour/session.cc

@@ -2173,6 +2173,9 @@ Session::resort_routes ()
 	*/
 
 	if (inital_connect_or_deletion_in_progress ()) {
+		/* drop any references during delete */
+		GraphEdges edges;
+		_current_route_graph = edges;
 		return;
 	}