Fix for segfaults when GeoIP has no region_name result

[keyist]

To replicate segfault:

GeoIP::City.new('/path/to/GeoLiteCity.dat').look_up('121.7.1.1')

with the database downloaded from http://geolite.maxmind.com/download/geoip/database/GeoLiteCity.dat.gz

(md5sum at time of writing: 6ed8c46880fc5579392375fbc5efa375)

[keyist]

Didn't see this already done at #8