netfilter: xt_TCPOPTSTRIP: don't use tcp_hdr()

In (bc6bcb59 netfilter: xt_TCPOPTSTRIP: fix possible mangling beyond
packet boundary), the use of tcp_hdr was introduced. However, we
cannot assume that skb->transport_header is set for non-local packets.

Cc: Florian Westphal <fw@strlen.de>
Reported-by: NPhil Oester <kernel@linuxace.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_TCPOPTSTRIP.c

@@ -48,11 +48,13 @@ tcpoptstrip_mangle_packet(struct sk_buff *skb,
 		return NF_DROP;
 
 	len = skb->len - tcphoff;
-	if (len < (int)sizeof(struct tcphdr) ||
-	    tcp_hdr(skb)->doff * 4 > len)
+	if (len < (int)sizeof(struct tcphdr))
 		return NF_DROP;
 
 	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
+	if (tcph->doff * 4 > len)
+		return NF_DROP;
+
 	opt  = (u_int8_t *)tcph;
 
 	/*