don't allow "" for device name

If a zero-length string is given, it is not rejected by
netlink in kernel so catch it at command line.
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>

ip/iplink.c

@@ -336,6 +336,8 @@ static int iplink_modify(int cmd, unsigned int flags, int argc, char **argv)
 
 	if (name) {
 		len = strlen(name) + 1;
+		if (len == 1)
+			invarg("null string is not valid name\n");
 		if (len > IFNAMSIZ)
 			invarg("\"name\" too long\n", *argv);
 		addattr_l(&req.n, sizeof(req), IFLA_IFNAME, name, len);