tfgeoip: Handle the failed lookup case gracefully

When a GeoIP lookup fails, do not try to use the result, as trying to
append NULL to a GString results in a GLib warning, which is not nice.

Reported-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #97 from algernon/h/geoip/fail-gracefully

tfgeoip: Handle the failed lookup case gracefully