bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Gregory Heytings]

> I simplified it down to:
>
> (with-current-buffer (get-buffer-create "*bug*")
>  (erase-buffer)
>  (insert (make-string 266666 ?n))
>  (goto-char (point-min))
>  (looking-at "[[:alpha:]]*=*"))
>
> This fails with 266666, and succeeds with 266665.

Even shorter:

(with-current-buffer (get-buffer-create "*bug*")
  (erase-buffer)
  (insert (make-string 266666 ?x))
  (goto-char (point-min))
  (looking-at "x*=*"))

Again this fails with 266666, and succeeds with 266665.

Likewise:

(with-current-buffer (get-buffer-create "*bug*")
  (erase-buffer)
  (insert (make-string 266666 ?x))
  (insert "=")
  (goto-char (point-min))
  (looking-at "x*=*"))

fails with 266666 and succeeds with 266665.