sit: fix an oops when IFLA_IPTUN_PROTO is not set

The use of this attribute has been added in 32b8a8e5 (sit: add IPv4 over
IPv4 support). It is optional, by default proto is IPPROTO_IPV6.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1296,7 +1296,7 @@ static int ipip6_validate(struct nlattr *tb[], struct nlattr *data[])
 {
 	u8 proto;
 
-	if (!data)
+	if (!data || !data[IFLA_IPTUN_PROTO])
 		return 0;
 
 	proto = nla_get_u8(data[IFLA_IPTUN_PROTO]);