use dk_log2_size=3 in empty dicts to avoid UBSan

Author: picnixz

Objects/dictobject.c

@@ -588,10 +588,13 @@ estimate_log2_keysize(Py_ssize_t n)
 
 /* This immutable, empty PyDictKeysObject is used for PyDict_Clear()
  * (which cannot fail and thus can do no allocation).
+ *
+ * See https://github.com/python/cpython/pull/127568#discussion_r1868070614
+ * for the rationale of why dk_log2_size is 3 instead of 0 for empty dicts.
  */
 static PyDictKeysObject empty_keys_struct = {
         _Py_DICT_IMMORTAL_INITIAL_REFCNT, /* dk_refcnt */
-        0, /* dk_log2_size */
+        3, /* dk_log2_size */
         0, /* dk_log2_index_bytes */
         DICT_KEYS_UNICODE, /* dk_kind */
 #ifdef Py_GIL_DISABLED