bug#7496: 23.2; copy recursive keymap cause crash

[Drew Adams]

> It's easy to reproduce, though: Just eval this form and Emacs will
> segfault (if it's older than yesterday's):
> 
> (let ((map (make-sparse-keymap)))
>   (define-key map " " map)
>   (copy-keymap map))

FWIW, on MS Windows, Emacs 26.3, emacs -Q hangs if I
do that, after showing this message in the echo area:

  Re-entering top level after C stack overflow.

Have to kill the process with the Task Manager.