ipfw denial of service

In some cases, the `tcp-setmss` handler may free the packet data and throw an error without halting the rule processing engine. A subsequent rule can then allow the traffic after the packet data is gone, resulting in a NULL pointer dereference. Maliciously crafted packets sent from a remote host may result in a Denial of Service (DoS) if the `tcp-setmss` directive is used and a subsequent rule would allow the traffic to pass.