Fix crash at session close - fix use-after-free

Pianoroll::maybe_update() was able to use _session
after session was already unset.

gtk2_ardour/pianoroll.cc

@@ -1865,6 +1865,13 @@ Pianoroll::set_session (ARDOUR::Session* s)
 	}
 }
 
+void
+Pianoroll::session_going_away ()
+{
+	CueEditor::session_going_away ();
+	_update_connection.disconnect ();
+}
+
 void
 Pianoroll::map_transport_state ()
 {

gtk2_ardour/pianoroll.h

@@ -148,6 +148,7 @@ class Pianoroll : public CueEditor
 	bool key_release_handler (ArdourCanvas::Item*, GdkEvent*, ItemType);
 
 	void escape ();
+	void session_going_away ();
 
  private:
 	ArdourCanvas::Ruler*     bbt_ruler;