Test smallest allocation of large type (test_malloc)

Test smallest allocation of large type (test_malloc)
(Cherry-pick commits 29d91db, 937b173 from 'master' branch.)

* src/atomic_ops_malloc.c (AO_malloc_large): Add assertion that the
stored size is greater than LOG_MAX_SIZE.
* src/atomic_ops_malloc.c (AO_malloc): Add assertions for log_size.
* tests/test_malloc.c (LOG_MAX_SIZE, CHUNK_SIZE): New macro (copied
from atomic_ops_malloc.c).
* tests/test_malloc.c (main): Call AO_free(0), AO_malloc(0); add comment.
* tests/test_malloc.c [HAVE_MMAP] (main): Call
AO_malloc(CHUNK_SIZE-sizeof(AO_t)+1).