Set correct default probe_asn and probe_cc

[bassosimone]

When porting mkall-ios to MK v0.10.5, I needed to write this code:

MKUTIL_WRAP_GET_POINTER(MKGeoIPLookupResults, perform,
                        [](auto settings) noexcept {
  auto results = new mk::geoip::LookupResults{mk::geoip::lookup(*settings)};
  if (!results->good) {
    // Implementation note: the logic used by mk::geoip::lookup to determine
    // success checks the values of the probe_ip, probe_asn, etc fields. This
    // means we cannot initialize the results to the values we want to have
    // back on failure ("ZZ", "AS0", etc). We should instead check whether we
    // are good and set them afterwards. This glue code can probably go, if
    // we include this logic into the next release of mkgeoip.
    //
    // This code has been adapted from MK v0.10.4's geoiplookup.c. We should
    // actually fix geoiplookup to avoid duplicating code.
    if (results->probe_asn_string.empty()) {
      results->probe_asn_string = "AS0";
    }
    if (results->probe_cc.empty()) {
      results->probe_cc = "ZZ";
    }
  }
  return results;
})

To avoid duplicating code between mkall-ios and android-libs, it would actually be much better if we could put this logic into this repository instead.