auth: fix geoip_mmdb backend MMDB_open error handling

auth: fix geoip_mmdb backend MMDB_open error handling

According to [MMDB_open docs], errors from `MMDB_open` should be
compared to `MMDB_SUCCESS` (which is 0). All other errors are > 0, which
means that existing check never detects errors, causing further queries
to fail with error:
```
The MaxMind DB file is in a format this library can't handle
(unknown record size or binary format version)
```

[MMDB_open docs]: https://maxmind.github.io/libmaxminddb/#mmdb_open

Signed-off-by: Ensar Sarajčić <dev@ensarsarajcic.com>