Harden FastMalloc against partial pointer overflows
https://bugs.webkit.org/show_bug.cgi?id=114716

Reviewed by Gavin Barraclough.

Bite the bullet and perform object alignment checks on free.
malloc/free micro benchmark shows a regression, but real
benchmarks don't.  There's a little code motion in this avoid
taking too much of a performance hit.  In addition to the
alignment check we also validate the containing span as
we've already taken the hit of finding it.

* wtf/FastMalloc.cpp:
(WTF::TCMalloc_ThreadCache::CreateCacheIfNecessary):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148587 268f45cc-cd09-0410-ab3c-d52691b4dbfc