ip6_tunnel: fix potential NULL pointer dereference

The function ip6_tnl_validate assumes that the rtnl
attribute IFLA_IPTUN_PROTO always be filled . If this
attribute is not filled by  the userspace application
kernel get crashed with NULL pointer dereference. This
patch fixes the potential kernel crash when
IFLA_IPTUN_PROTO is missing .
Signed-off-by: Susant Sahani <susant@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_tunnel.c

@@ -1557,7 +1557,7 @@ static int ip6_tnl_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]);