The existing implementation converts the deadline time to an int64,
but does not handle overflow. If the calculated deadline is negative
but the user specified deadline is in the future, then we can assume
the calculation overflowed, and set the deadline to math.MaxInt64.

Fixes #14431

Change-Id: I54dbb4f02bc7ffb9cae8cf62e4e967e9c6541ec6
Reviewed-on: https://go-review.googlesource.com/19758Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>