tcp: address a warning

t_state is an unsigned variable, so no need for testing that it is
non-negative.

Reported by: Coverity Scan
CID: 1390885
Reviewed by: glebius
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44619

(cherry picked from commit 6b454da6bbaa3327cf9b7185d198c96ffc1b88f4)