avoid possible use of uninitialized intf_addr in nmrp_do()
Silences a warning by gcc:
.
nmrp.c:462:7: warning: ‘intf_addr’ may be used uninitialized in this function [-Wmaybe-uninitialized]
462 | if (ethsock_ip_add(sock, intf_addr, ipmask.s_addr, &ip_undo) != 0) {
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~