Mark unallocated/freed memory as inaccessible using ASan functionality

Mark unallocated/freed memory as inaccessible using ASan functionality

Note: to disable it, AO_NO_MALLOC_POISON should be defined by client.

* src/atomic_ops_malloc.c [AO_ADDRESS_SANITIZER && !AO_NO_MALLOC_POISON]
(__asan_poison_memory_region, __asan_unpoison_memory_region): Prototype.
* src/atomic_ops_malloc.c (ASAN_POISON_MEMORY_REGION,
ASAN_UNPOISON_MEMORY_REGION): New macro.
* src/atomic_ops_malloc.c (add_chunk_as): Call
ASAN_POISON_MEMORY_REGION() for each pushed region.
* src/atomic_ops_malloc.c (AO_malloc): Call
ASAN_UNPOISON_MEMORY_REGION() for the returned memory region.
* src/atomic_ops_malloc.c (AO_free): Call ASAN_POISON_MEMORY_REGION()
before pushing the freed region to AO_free_list.