bug#30931: 27.0.50; Crash in "Automatic GC"

[Michał Kondraciuk]

I was able to reproduce crash in the same place in bare Emacs versions 
27.0.50, 25.3, 24.4 with the form below.

(progn
  (defun on-overlay-modification (&rest args)
    (push nil buffer-undo-list))

  (defun crash ()
    (save-restriction
      (narrow-to-region 1 1)

      (let (end)
        (insert " 123aaa")
        (narrow-to-region 2 3)

        (setq end (set-marker (make-marker) 3))
        (save-restriction
          (widen)
          (lisp-indent-line))

        (garbage-collect)

        (overlay-put (make-overlay end (1+ end))
                     'modification-hooks '(on-overlay-modification)))))

  (with-current-buffer "*scratch*"
    (let ((buffer-undo-list t))
      (while t
        (erase-buffer)
        (crash)))))