initialize complete memory segment in calloc

@@ -244,7 +244,7 @@ TR_calloc(size_t nmemb, size_t size)
 	size_t   _size = nmemb * size;
 	void   * mem   = TR_malloc(_size);
 
-	memset(mem, 0, _size);
+	memset(mem, 0, TR_getUsableSize(mem));
 
 	return mem;
 }