[patch] fix build on architectures with a signed size_t

[Florian Weimer]

* Andrew Haley:

>> I was more concerned with the reality on s390, not what's in the
>> standard.  Unfortunately, my first test case was wrong.
>
> Is the reality then that GNU C on s/390 is not ISO C compliant?
> I didn't know that.  It must break a ton of code.

Let me repeat: My initial test case was wrong.  It reported size_t as
signed, when it wasn't.  Sorry about that.

At least on raptor.debian.org, in the sid chroot, size_t is unsigned.