Set tun devname to IFNAMESIZ+1 to avoid warning

Relevant output from make:

../lib/tun.c: In function ‘tun_new’:
../lib/tun.c:688: warning: array subscript is above array bounds

Caused by assignement:
(*tun)->devname[IFNAMSIZ] = 0;
while devname defined as:
char devname[IFNAMSIZ];

Signed-off-by: Emmanuel Bretelle <chantra@debuntu.org>

lib/tun.h

@@ -45,7 +45,7 @@ struct tun_t {
   struct in_addr netmask;
   int addrs;             /* Number of allocated IP addresses */
   int routes;            /* One if we allocated an automatic route */
-  char devname[IFNAMSIZ];/* Name of the tun device */
+  char devname[IFNAMSIZ+1];/* Name of the tun device */
   int (*cb_ind) (struct tun_t *tun, void *pack, unsigned len);
 };