bug#56623: memory leak introduced by new nonrecursive lisp reader

[Greg Hendershott <mail@greghendershott.com>]

I think I've discovered the problem.

The new `read' leaks when it errors.

Here's a little driver to elicit this:

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;; Run this with e.g. emacs --batch -Q -l example.el
(message "Run htop and observe the RES value")
(dotimes (n 512)
  (message "%s" n)
  (with-temp-buffer
    ;; Insert an unclosed sexp
    (insert "(")
    ;; Make it large, with quite a few sub-expressions to read
    (dotimes (_ 8192)
      (insert (format "%S" '(foo bar baz "a somewhat long string blah blah blah blah blah\n"))))
    ;; Go to start and attempt a read. This will fail due to the
    ;; unclosed paren, so we use `ignore-errors'.
    (goto-char (point-min))
    (ignore-errors (read (current-buffer)))))

;; With the old recursive lread.c, the RES value will oscillate around
;; a stable low value (presumably as GC happens).

;; With the new non-recursive lread.c, the RES value will climb
;; unbounded into hundreds of megabytes.

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I hope this helps!