tcp: fix handling of TCP RST segments missing timestamps

A TCP RST segment should be processed even it is missing TCP
timestamps.

Reported by: dmgk@, kevans@
Reviewed by: rscheff@, dmgk@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D28143

(cherry picked from commit cc3c34859eab1b317d0f38731355b53f7d978c97)