tzcode: Fix overflow handling in TZ parser.

Obtained from: upstream 9fc11a27
MFC after: 1 week
PR: 276281
Approved by: re (cperciva)

(cherry picked from commit 1365bb722af1365baee6ea1e3d44917533908d53)
(cherry picked from commit beb958dca02abf79a2172e702c2d24bbccde60fa)