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

[DJ Delorie]

Jonathan Nieder <jrnieder@gmail.com> writes:
> nit: unsigned overflow is defined in C to wrap around, so we don't
> have to worry about undefined behavior here.

Including implied overflow by type cast conversions?

> 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.

The only two places they could be called from is mallopt() and the
tunables macros in arena.c.  The tunables code ignores any return value.