Fix TCP reassembly vulnerability.

Security:	FreeBSD-SA-14:08.tcp
Security:	CVE-2014-3000
Approved by:	so


git-svn-id: svn+ssh://svn.freebsd.org/base/releng/8.3@265125 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f