[JENKINS-29357] Fix findbugs issue in build-timeout-plugin

Findbugs are introduced in https://github.com/jenkinsci/build-timeout-plugin/pull/41.
It reveals possible problems in DeadlineTimeStrategy (~~JENKINS-29163~~).
I'll fix that.

ikedam created issue - 2015-07-11 00:32

SCM/JIRA link daemon made changes - 2015-07-18 04:34

Resolution		New: Fixed [ 1 ]
Status	Original: Open [ 1 ]	New: Resolved [ 5 ]

R. Tyler Croy made changes - 2016-07-25 23:57

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