[NETFILTER]: H.323 helper: fix possible NULL-ptr dereference

An RCF message containing a timeout results in a NULL-ptr dereference if
no RRQ has been seen before.

Noticed by the "SATURN tool", reported by Thomas Dillig <tdillig@stanford.edu>
and Isil Dillig <isil@stanford.edu>.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_helper_h323.c

@@ -1200,7 +1200,7 @@ static struct ip_conntrack_expect *find_expect(struct ip_conntrack *ct,
 	tuple.dst.protonum = IPPROTO_TCP;
 
 	exp = __ip_conntrack_expect_find(&tuple);
-	if (exp->master == ct)
+	if (exp && exp->master == ct)
 		return exp;
 	return NULL;
 }