Make sure the arena cache cannot be reused.

The arena cache is freed when the jemalloc key is destroyed. However,
if another key destroy function winds up making an allocation, jemalloc
can wind up rewriting the arena pointers into the previous arena cache
pointer. The fix is to clear out the pointer and make sure that
the arena cache will not be recreated.

Bug: 20296122
(cherry picked from commit 10e814ba3f01361999d824f0c67db179580d4ca2)

Change-Id: Ib1bb15144eba369df36e81e967fbc9c65f94aa4e
1 file changed