[sanitizer] Make sure libmalloc doesn't remove the sanitizer zone from malloc_zones[0]

In certain OS versions, it was possible that libmalloc replaced the sanitizer zone from being the default zone (i.e. being in malloc_zones[0]). This patch introduces a failsafe that makes sure we always stay the default zone. No testcase for this, because this doesn't reproduce under normal circumstances.

Differential Revision: https://reviews.llvm.org/D27083

llvm-svn: 289376