Work around tzcode's reliance on signed overflow.

I've mailed the tz list about this, and will switch to whatever upstream
fix comes along as soon as it's available.

Bug: 10310929

(cherry picked from commit 7843d44a594270bcb56e98b130603c054f8a9d38)

Change-Id: I205e2440703444c50cecd91d3458d33613ddbc59
2 files changed