Fix error handling when setting up the UDP socket.

Due to this typo, if tinc managed to set up the TCP socket but not the
UDP socket, it would continue anyway.

The regression was introduced in
6bc5d626a8.

src/net_setup.c

@@ -676,7 +676,7 @@ static bool add_listen_address(char *address, bool bindto) {
 
 		int udp_fd = setup_vpn_in_socket((sockaddr_t *) aip->ai_addr);
 
-		if(tcp_fd < 0) {
+		if(udp_fd < 0) {
 			close(tcp_fd);
 			continue;
 		}