[tz] Checked arithmetic bug

[Dag-Erling Smørgrav]

In at least one instance around line 1187 of localtime.c, the code
assumes that increment_overflow_time() does not modify the result if the
operation overflowed, but that is not true when using C23 checked
arithmetic.

Details: https://bugs.freebsd.org/276281, particularly comment 11.

One possible solution is attached.  I'm not overly fond of it because it
penalizes the common case, but it was easier than trying to understand
and reformulate the loop in tzparse().

DES
-- 
Dag-Erling Smørgrav - des at des.no

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