Fix an infinite loop in tcp_tw_2msl_scan() when an INP_TIMEWAIT inp has
been destroyed before its tcptw with INVARIANTS undefined.

This is a symmetric change of r307551:

A INP_TIMEWAIT inp should not be destroyed before its tcptw, and INVARIANTS
will catch this case. If INVARIANTS is undefined it will emit a log(LOG_ERR)
and avoid a hard to debug infinite loop in tcp_tw_2msl_scan().

Reported by: Ben Rubson, hselasky
Submitted by: hselasky
Tested by: Ben Rubson, jch
MFC after: 1 week
Sponsored by: Verisign, inc
Differential Revision: https://reviews.freebsd.org/D12267