bug#874: Args out of range with new auto-composition implementation

[Juanma Barranquero]

;;; testbug.el ;;;
(column-number-mode 1)

(defun testbug ()
  (interactive)
  (split-window)
  (other-window 1)
  (pop-to-buffer (get-buffer-create "testbug")))
;;;;;;;;;;;;;;;;;

emacs -Q -l testbug.el
C-h H
M-: (goto-char 276) <RET>   ;; or move the cursor to any auto-composed char
M-x testbug <RET>

  => "Args out of range: 274, 274"

(and the windows are not correctly painted.)

At home's laptop I can reproduce the bug at will; as a weird
datapoint, at work I've only been able to reproduce it when the buffer
is called "*ielm*", even if IELM is *not* loaded.

emacs-devel discussion (including a backtrace):

http://lists.gnu.org/archive/html/emacs-devel/2008-09/msg00045.html