Session drop graph-nodes before ~Session

Previously active Routes were retained until the end of
Session d'tor and not dropped during Session::destroy.

While most resources were explicitly cleaned up via DropReferences,
Processor UIs are kept around until the actual destructor runs.
Likewise some controllable are kept around while the GraphNode (Route)
owning it is not released.

libs/ardour/session.cc

@@ -690,6 +690,8 @@ Session::destroy ()
 
 	/* drop GraphNode references */
 	_graph_chain.reset ();
+	_current_route_graph = GraphEdges ();
+
 	_io_graph_chain[0].reset ();
 	_io_graph_chain[1].reset ();