py/objtype: Inherit protocol vtable from base class only if it exists.

py/objtype.c

@@ -918,7 +918,9 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
     // abstract base class which would translate C-level protocol to
     // Python method calls, and any subclass inheriting from it will
     // support this feature.
+    if (len > 0) {
         o->protocol = ((mp_obj_type_t*)MP_OBJ_TO_PTR(items[0]))->protocol;
+    }
     o->bases_tuple = MP_OBJ_TO_PTR(bases_tuple);
     o->locals_dict = MP_OBJ_TO_PTR(locals_dict);