Fix crash when cleaning up w/o destroying session

- Control-protocols may transmit data during cleanup
 (e.g. reset surface), and need the Audio-engine to do so.
- destroying the ControlProtocolManager w/o the Session calling
 ::drop_protocols(), lead to a double free.

libs/ardour/control_protocol_manager.cc

@@ -76,6 +76,7 @@ ControlProtocolManager::~ControlProtocolManager()
 
 
 	for (list<ControlProtocolInfo*>::iterator p = control_protocol_info.begin(); p != control_protocol_info.end(); ++p) {
+		(*p)->protocol = 0; // protocol was already destroyed above.
 		delete (*p);
 	}
 

libs/ardour/globals.cc

@@ -597,13 +597,13 @@ ARDOUR::cleanup ()
 		return;
 	}
 
+	delete &ControlProtocolManager::instance();
 	ARDOUR::AudioEngine::destroy ();
 
 	delete Library;
 #ifdef HAVE_LRDF
 	lrdf_cleanup ();
 #endif
-	delete &ControlProtocolManager::instance();
 #ifdef WINDOWS_VST_SUPPORT
 	fst_exit ();
 #endif