Do not use ampersand for function call return values.

Our PHP hoster reported the following problem:

[Sun Oct 19 08:08:40 2014] [warn] [client 66.249.78.166] mod_fcgid:
stderr:
PHP Strict Standards:  Only variables should be assigned by reference in
/…/wp-datenlandkarten/tool/lib/geo.php on line 609

To avoid trashing the log file, this commit was necessary.