Remove a bogus leftover use of a valid pointer as a throwaway variable

in getinterfaces. This was the cause of an overlapping memcpy reported
at http://seclists.org/nmap-dev/2009/q2/0713.html.

In the code, sin is a pointer that holds the address of the IP address
configured for an interface. It is copied into a tmpifr.ifr_addr before
each ioctl, perhaps because that is required on some platforms even
though on Linux only ifr_name is needed by the ioctl. When the ioctl
returns, it overwrites whatever was in ifr_addr because that member is
in a union, so sin is kept in order to restore the address again before
the next ioctl.

In the code that handles SIOCGIFNETMASK, sin was mistakenly used as a
temporary pointer and redirected to &tmpifr.ifr_addr. This caused all
future memcpys before ioctl to copy tmpifr.ifr_addr to itself, rather
than copying in the IP address of the interface.

The throwaway sin assignment was not even used; the code that used it
was modified in r2751. So now we just keep sin pointing where it should
the whole time.

tcpip.cc

@@ -2995,7 +2995,8 @@ static struct interface_info *getinterfaces_siocgifconf(int *howmany) {
     else if (rc < 0)
       mydevs[numifaces].netmask_bits = 32;
     else {
-      sin = (struct sockaddr_in *) &(tmpifr.ifr_addr); /* ifr_netmask only on Linux */
+      /* We would use ifr_netmask, but that's only on Linux, so use ifr_addr
+         which shares the same memory space in a union. */
       addr_stob(&(tmpifr.ifr_addr), &mydevs[numifaces].netmask_bits);
     }