Avoid 500 server error with IPv6 addresses (GeoIP)

I get 500 Server errors when searching for resources with a non-empty string.
With `DEBUG=True` I can get the exception: `Invalid database type; expected IPv4 address` which is backtraced to the `getcountry_code` function in the `stats/geoip.py` file.

The proposed try/except patch fixes the issue by returning the default empty string if geoip cannot find the country.

As far as I know, this is used for statistic purposes, so providing the country guessed by the IP it is not crucial for metashare.