bug#7537: 23.2.90; infinite loop in scrolling

[Leo]

An unstoppable infinite loop can be reproduced by

1. emacs -Q
2. Evaluate the following expression in *scratch* with C-x C-e.
   (progn
     (setq scroll-conservatively 100)
     (setq scroll-preserve-screen-position 'always)
     (goto-char (point-min))
     (set-window-vscroll nil 1 t))

Tested in Emacs build with X11 on OSX.

See: http://lists.gnu.org/archive/html/emacs-devel/2010-12/msg00073.html

Leo