tcp hpts: initialize variable

Ensure that tv.tv_sec is zero in all code paths.

Reported by: Coverity Scan
CID: 1527724
Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44584

(cherry picked from commit aaaa01c0c858fd703194c6cbd515dd514574381f)