[tz] localtime crash and fix

[Paul Eggert]

On 2023-11-24 08:22, Alois Treindl via tz wrote:
> The use of union to save memory is dangerous, in my opinion. There is no 
> need in today's machines to save a few kilobytes of RAM for a process.
> Not even in embedded software for watches.

When the memory is on the stack it still makes sense in some cases to 
save even a few kilobytes of RAM, as highly-threaded apps often have 
surprisingly small stacks. And anyway, the union has nothing to do with 
this particular bug.

Thanks for reporting the problem. I installed the attached patches. The 
first patch fixes the bug; the second is a minor cleanup I noticed while 
in the neighborhood.

The first patch clears just two slots rather than the entire struct, to 
avoid masking other errors.
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