Fix signed integer wrap in time2sub()

There is a signed integer wrap that has an undefined behevior and
GCC 7 knows about it that the compiled code just crashes.

Fix can be verified by building with -fsanitize=undefined option.

<https://jira.mongodb.org/browse/CDRIVER-2010

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Signed-off-by: Petr Písař <ppisar@redhat.com>
Signed-off-by: A. Jesse Jiryu Davis <jesse@mongodb.com>