In Fposn_at_x_y do not allow internal window as argument.

* keyboard.c (Fposn_at_x_y): Do not allow internal window as
argument.

src/ChangeLog

+2012-08-23  Martin Rudalics  <rudalics@gmx.at>
+
+	* keyboard.c (Fposn_at_x_y): Do not allow internal window as
+	argument.
+
 2012-08-23  Paul Eggert  <eggert@cs.ucla.edu>
 
 	* atimer.c, atimer.h (turn_on_atimers): Use bool for boolean.

src/keyboard.c

@@ -11251,7 +11251,7 @@ The `posn-' functions access elements of such lists.  */)
 
   if (WINDOWP (frame_or_window))
     {
-      struct window *w = decode_valid_window (frame_or_window);
+      struct window *w = decode_live_window (frame_or_window);
 
       XSETINT (x, (XINT (x)
 		   + WINDOW_LEFT_EDGE_X (w)