util/time test failure on Jenkins

Our 'util/time' test fails on our tor-ci-mingwcross-master-test builder with:

util/time: Mar 27 13:29:43.773 [warn] tor_gmtime_r(): Bug: gmtime(-1) failed with error Invalid argument: Rounding up to 1970 (on Tor 0.4.1.0-alpha-dev )
 
   [time FAILED]

See: https://jenkins.torproject.org/job/tor-ci-mingwcross-master-test/ARCHITECTURE=amd64,SUITE=stretch/lastBuild/console

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