[NETFILTER]: nf_conntrack_netbios_ns: fix uninitialized member in expectation

->helper is uninitialized in the expectation registered by the netbios_ns
helper and it later copied to the expected connection, which causes invalid
memory dereferences when trying to call the helper.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_netbios_ns.c

@@ -89,6 +89,7 @@ static int help(struct sk_buff **pskb, unsigned int protoff,
 
 	exp->expectfn             = NULL;
 	exp->flags                = NF_CT_EXPECT_PERMANENT;
+	exp->helper               = NULL;
 
 	nf_conntrack_expect_related(exp);
 	nf_conntrack_expect_put(exp);