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: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>