vm_setivar_default: trigger write barrier on the fields_obj, not obj

Author: byroot

vm_insnhelper.c

@@ -1482,7 +1482,7 @@ vm_setivar_default(VALUE obj, ID id, VALUE val, shape_id_t dest_shape_id, attr_i
 
     VALUE fields_obj = rb_obj_fields(obj, id);
     RUBY_ASSERT(fields_obj);
-    RB_OBJ_WRITE(obj, &rb_imemo_fields_ptr(fields_obj)[index], val);
+    RB_OBJ_WRITE(fields_obj, &rb_imemo_fields_ptr(fields_obj)[index], val);
 
     if (shape_id != dest_shape_id) {
         RBASIC_SET_SHAPE_ID(obj, dest_shape_id);