malloc: Add another test for tcache double free check.

[DJ Delorie]

"Carlos O'Donell" <carlos@redhat.com> writes:
> Did you test that the test crashes without Florian's fix, but
> with your doubl-free code enabled?

Yes.  That took a while, and the result was the memset in the test.  I
spent a while trying to get the "invalid bin" to be outside mapped
space, to no avail.

"Carlos O'Donell" <carlos@redhat.com> writes:
> Needs a bug number in ChangeLog:
> 	[BZ #23907]

Done.

>> +  /* This is just under the MMAP threshold.  */
>
> s/MMAP/mmap/g

Fixed.

> s/RHBZ#1652495/BZ #23907/g

Fixed.