bug#10939: 24.0.94; backward-delete-char-untabify read-only error

[Leo]

1. (setq backward-delete-char-untabify-method 'hungry)
2. (setq enable-recursive-minibuffers t)
3. M-: and then type some spaces
4. M-x backward-delete-char-untabify

You should see something like:
  delete-backward-char: Text is read-only

This patch fixes it:

diff --git a/lisp/simple.el b/lisp/simple.el
index c14d0d70..0faf54bc 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -3458,8 +3458,10 @@ (defun backward-delete-char-untabify (arg &optional 
killp)
                      ((eq backward-delete-char-untabify-method 'all)
                       " \t\n\r")))
          (n (if skip
-                (let ((wh (- (point) (save-excursion (skip-chars-backward skip)
-                                                     (point)))))
+                (let* ((oldpt (point))
+                       (wh (- oldpt (save-excursion
+                                      (skip-chars-backward skip)
+                                      (constrain-to-field nil oldpt)))))
                   (+ arg (if (zerop wh) 0 (1- wh))))
               arg)))
     ;; Avoid warning about delete-backward-char