[bug-inetutils] Potential segmentation fault in ifconfig

[Diego Nieto Cid]

Here is a trace of the execution path leading to the segmentation fault:

| void parse_cmdline (int argc, char *argv[]) {
|  if (!ifs) {
|  /* No interfaces specified.  Get a list of all interfaces.  */
|  struct if_nameindex *ifnx, *ifnxp;
|  ifnx = ifnxp = if_nameindex ();
|
|      struct if_nameindex * if_nameindex (void) {
|        #if defined(SIOCGIFCONF)
|        int fd = socket (AF_INET, SOCK_DGRAM, 0);
|        [...]
|        if (fd < 0)
|        return NULL;
|
|  while (ifnxp->if_index != 0 || ifnxp->if_name != NULL)

This path is reproducible when running on the Hurd while the pfinet
translator have not been started for eth0 as in that case the socket()
call returns -1. But that's another story :)

The point is that parse_cmdline should check for error conditions
signaled by if_nameindex (a NULL return value) and take appropriate
actions when that is the case.