Fix crash when pressing "Esc" without a session

Ardour's UI can process a key-binding or emit the Escape() signal
without a loaded session or when unloading a session.

gtk2_ardour/editor_mouse.cc

@@ -2562,7 +2562,7 @@ Editor::escape ()
 {
 	if (_drags->active ()) {
 		_drags->abort ();
-	} else {
+	} else if (_session) {
 		selection->clear ();
 
 		/* if session is playing a range, cancel that */