relay: Refactor address and port checks

tor_addr_port_is_valid_ap(ap, 0) checks if the address or port are
zero, exactly like the previous code.

Preparation for 33817.

src/feature/relay/circuitbuild_relay.c

@@ -139,7 +139,7 @@ circuit_extend_lspec_valid_helper(const struct extend_cell_t *ec,
     return -1;
   }
 
-  if (!ec->orport_ipv4.port || tor_addr_is_null(&ec->orport_ipv4.addr)) {
+  if (!tor_addr_port_is_valid_ap(&ec->orport_ipv4, 0)) {
     log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
            "Client asked me to extend to zero destination port or addr.");
     return -1;