hurd: Fix buffer overrun in __if_nametoindex

and building with mainline GCC which reports it.

	* sysdeps/mach/hurd/if_index.c (__if_nametoindex): Always end
	ifr.fr_name with a NUL caracter.

ChangeLog

@@ -1,3 +1,8 @@
+2018-04-03  Samuel Thibault  <samuel.thibault@ens-lyon.org>
+
+	* sysdeps/mach/hurd/if_index.c (__if_nametoindex): Always end
+	ifr.fr_name with a NUL caracter.
+
 2018-04-03  Wilco Dijkstra  <wdijkstr@arm.com>
 
 	* sysdeps/ieee754/dbl-64/s_sin.c (__sin): Cleanup ifdefs.

sysdeps/mach/hurd/if_index.c

@@ -37,7 +37,9 @@ __if_nametoindex (const char *ifname)
   if (fd < 0)
     return 0;
 
-  strncpy (ifr.ifr_name, ifname, IFNAMSIZ);
+  strncpy (ifr.ifr_name, ifname, IFNAMSIZ - 1);
+  ifr.ifr_name[IFNAMESIZ - 1] = '\0';
+
   if (__ioctl (fd, SIOCGIFINDEX, &ifr) < 0)
     {
       int saved_errno = errno;