disable invariant_check functions when invariant checks are disabled

src/bt_peer_connection.cpp

@@ -3337,7 +3337,7 @@ namespace libtorrent
 		}
 	}
 
-#ifdef TORRENT_DEBUG
+#if defined TORRENT_DEBUG && !defined TORRENT_DISABLE_INVARIANT_CHECKS
 	void bt_peer_connection::check_invariant() const
 	{
 		boost::shared_ptr<torrent> t = associated_torrent().lock();