FIX: crash during city street building

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7280 8aca7d54-2c30-db11-9de9-000461428c89
FIX: city road building checks to see whether levelled tile has been flooded

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7281 8aca7d54-2c30-db11-9de9-000461428c89