Fix a memory corruption bug in chunk_alloc_dss().

Fix a memory corruption bug in chunk_alloc_dss() that was due to
claiming newly allocated memory is zeroed.

Reverse order of preference between mmap() and sbrk() to prefer mmap().

Clean up management of 'zero' parameter in chunk_alloc*().

src/chunk_dss.c

@@ -89,7 +89,6 @@ chunk_alloc_dss(size_t size, size_t alignment, bool *zero)
 				malloc_mutex_unlock(&dss_mtx);
 				if (cpad_size != 0)
 					chunk_dealloc(cpad, cpad_size, true);
-				*zero = true;
 				return (ret);
 			}
 		} while (dss_prev != (void *)-1);