PyType_IsSubtype(): test tp_flags for HAVE_CLASS bit before accessing

a->tp_mro.  If a doesn't have class, it's considered a subclass only
of itself or of 'object'.

This one fix is enough to prevent the ExtensionClass test suite from
dumping core, but that doesn't say much (it's a rather small test
suite).  Also note that for ExtensionClass-defined types, a different
subclass test may be needed.  But I haven't checked whether
PyType_IsSubtype() is actually used in situations where this matters
-- probably it doesn't, since we also don't check for classic classes.

Objects/typeobject.c

@@ -273,6 +273,9 @@ PyType_IsSubtype(PyTypeObject *a, PyTypeObject *b)
 {
 	PyObject *mro;
 
+	if (!(a->tp_flags & Py_TPFLAGS_HAVE_CLASS))
+		return b == a || b == &PyBaseObject_Type;
+
 	mro = a->tp_mro;
 	if (mro != NULL) {
 		/* Deal with multiple inheritance without recursion