[patch] Correct range checking in mallopt/mxfast/tcache [BZ #25194]

[Jonathan Nieder]

DJ Delorie wrote:

> Subject: Correct range checking in mallopt/mxfast/tcache [BZ #25194]
>
> do_set_tcache_max, do_set_mxfast:
> Fix two instances of comparing "size_t < 0"
> Both cases have upper limit, so the "negative value" case
> is already handled via (undefined) overflow semantics.

nit: unsigned overflow is defined in C to wrap around, so we don't
have to worry about undefined behavior here.

This also changes the return value from these helpers.  Forgive my
ignorance: where do they get called?  I assumed it would be
__libc_mallopt, but I don't see any calls from there.

Thanks,
Jonathan