HACK: do not crash more than once when _cached.xml is corrupt

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7384 8aca7d54-2c30-db11-9de9-000461428c89
FIX: to avoid crashed with NULL pointer in skripts return good NONE

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7385 8aca7d54-2c30-db11-9de9-000461428c89