router: Refactor IPv6 ORPort function logic

Return early when there is no suitable IPv6 ORPort.
Show the address and port on error, using a convenience function.

Code simplification and refactoring.

Cleanup after 32588.

src/feature/relay/router.c

@@ -1460,20 +1460,21 @@ router_get_advertised_ipv6_or_ap(const or_options_t *options,
     return;
   }
 
-  /* Like IPv4, if the relay is configured using the default
-   * authorities, disallow internal IPs. Otherwise, allow them. */
+  /* If the relay is configured using the default authorities, disallow
+   * internal IPs. Otherwise, allow them. For IPv4 ORPorts and DirPorts,
+   * this check is done in resolve_my_address(). See #33681. */
   const int default_auth = using_default_dir_authorities(options);
-  if (! tor_addr_is_internal(addr, 0) || ! default_auth) {
-    tor_addr_copy(&ipv6_ap_out->addr, addr);
-    ipv6_ap_out->port = port;
-  } else {
-    char addrbuf[TOR_ADDR_BUF_LEN];
+  if (tor_addr_is_internal(addr, 0) && default_auth) {
     log_warn(LD_CONFIG,
-             "Unable to use configured IPv6 address \"%s\" in a "
+             "Unable to use configured IPv6 ORPort \"%s\" in a "
              "descriptor. Skipping it. "
              "Try specifying a globally reachable address explicitly.",
-             tor_addr_to_str(addrbuf, addr, sizeof(addrbuf), 1));
+             fmt_addrport(addr, port));
+    return;
   }
+
+  tor_addr_copy(&ipv6_ap_out->addr, addr);
+  ipv6_ap_out->port = port;
 }
 
 /** Return the port that we should advertise as our DirPort;