bmalloc: eager scavenge leaves behind a bogus allocator

bmalloc: eager scavenge leaves behind a bogus allocator
https://bugs.webkit.org/show_bug.cgi?id=136743

Reviewed by Sam Weinig.

Be sure to clear the allocator after logging it in the eager scavenge
case, so that we don't later try to allocate out of the lines that we
have thrown away.

We didn't need to do this previously because scavenge would only happen
at thread exit time, after which no further allocation from the per-thread
cache would take place.

* bmalloc/Allocator.cpp:
(bmalloc::Allocator::scavenge):
* bmalloc/MediumAllocator.h:
(bmalloc::MediumAllocator::clear):
* bmalloc/SmallAllocator.h:
(bmalloc::SmallAllocator::clear):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173524 268f45cc-cd09-0410-ab3c-d52691b4dbfc