FIX: crash when building powertunnel

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6440 8aca7d54-2c30-db11-9de9-000461428c89
FIX: maintenance when removing tunnel tiles

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6441 8aca7d54-2c30-db11-9de9-000461428c89