FIX: no crash on brioken tunnels

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6556 8aca7d54-2c30-db11-9de9-000461428c89
Debug code for freelist

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6557 8aca7d54-2c30-db11-9de9-000461428c89