FIX another potential overflow

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6946 8aca7d54-2c30-db11-9de9-000461428c89
make clang happy

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6947 8aca7d54-2c30-db11-9de9-000461428c89