[patch] fix build on architectures with a signed size_t

[Florian Weimer]

* Andrew Haley:

> Florian Weimer wrote:
>> * Florian Weimer:
>> 
>>> * Matthias Klose:
>>>
>>>> attached are three patches to build on architectures with a signed size_t
>>>> (s390-linux-gnu).
>>> Yuck.  Is this really true?  Seems so.
>> 
>> No, it's not.
>
> It's not.

I was more concerned with the reality on s390, not what's in the
standard.  Unfortunately, my first test case was wrong.