[NETFILTER] ctnetlink: check if protoinfo is present

This fixes an oops triggered from userspace. If we don't pass information
about the private protocol info, the reference to attr will be NULL. This is
likely to happen in update messages.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_proto_tcp.c

@@ -362,6 +362,11 @@ static int nfattr_to_tcp(struct nfattr *cda[], struct ip_conntrack *ct)
 	struct nfattr *attr = cda[CTA_PROTOINFO_TCP-1];
 	struct nfattr *tb[CTA_PROTOINFO_TCP_MAX];
 
+	/* updates could not contain anything about the private
+	 * protocol info, in that case skip the parsing */
+	if (!attr)
+		return 0;
+
         nfattr_parse_nested(tb, CTA_PROTOINFO_TCP_MAX, attr);
 
 	if (!tb[CTA_PROTOINFO_TCP_STATE-1])