disable invariant_check functions when invariant checks are disabled

src/peer_connection.cpp

@@ -5573,7 +5573,7 @@ namespace libtorrent
 		setup_send();
 	}
 
-#ifdef TORRENT_DEBUG
+#if defined TORRENT_DEBUG && !defined TORRENT_DISABLE_INVARIANT_CHECKS
 	struct peer_count_t
 	{
 		peer_count_t(): num_peers(0), num_peers_with_timeouts(0), num_peers_with_nowant(0), num_not_requested(0) {}