Fix duplicate check for "only allow internal addresses if we are on a network with nonstandard authorities"

We have this code in config.c:

  if (tor_addr_is_internal(&myaddr, 0)) {
    /* make sure we're ok with publishing an internal IP */
    if (!options->DirAuthorities && !options->AlternateDirAuthority) {
      /* if they are using the default authorities, disallow internal IPs
       * always. */
      log_fn(warn_severity, LD_CONFIG,
             "Address '%s' resolves to private IP address '%s'. "
             "Tor servers that use the default DirAuthorities must have "
             "public IP addresses.", hostname, addr_string);
      tor_free(addr_string);
      return -1;
    }
...

And we now have this code in router.c (since legacy/trac#17153 (moved)):

        /* Like IPv4, if the relay is configured using the default
         * authorities, disallow internal IPs. Otherwise, allow them. */
        const int default_auth = (!options->DirAuthorities &&
                                  !options->AlternateDirAuthority);
        if (! tor_addr_is_internal(&p->addr, 0) || ! default_auth) {
          ipv6_orport = p;
          break;
...

These two checks are similar and I'd prefer that they be merged when possible.