[asan] Fix stack pointers comparison in FakeStack

Unlucky naming top/bottom for stack bounds, has nothing to do with real
stack top. So top > botton is here, and opposite of
9be88929 assumption.

This is minimal fix in case cherry-picks is needed. Naming fix and
testing (if possible) will be in followup patches.

Introduced in 9be88929.
Can't symply reverted 9be88929 as
it fixes ~10 year old bug, accidentally exposed by
a8bef886.