[asan] Remove system_malloc_zone

This seems to be dead code – `system_malloc_zone` is initialized as null and we never assign to it. This code is apparently currently never executed (in ASan), but is causes trouble for the TSan OS X port. Let's replace the checks with `COMMON_MALLOC_ENTER` (`ENSURE_ASAN_INITED()`).

Differential Revision: http://reviews.llvm.org/D14334

llvm-svn: 252876