I cared about the nesting of each_object. Do you think it's OK?

Thank you for your review.

I have simulated it in my head again and I believe it is fine.
To enter an if block, it is limited to mrb->gc.iterating == TRUE.
To enter an else block, mrb->gc.iterating = TRUE is set and mrb->gc.iterating = FALSE is set when returning from mrb_objspace_each_objects().
My conclusion is that there is no need to remember the previous state by the iterating variable.

I also checked the issue in #3616, which was the impetus for the change to the current form, but could not reproduce the problem.