Fix null pointer dereference in xf86_reload_cursors().

Upon resume, X may try to dereference a null pointer, which has been
reported in Debian bug #507916 (http://bugs.debian.org/507916).

Jim Paris came up with a patch which solves the problem for him. Here's
a (hopefully) fixed version of his patch (without the typo).

Cc: Jim Paris <jim@jtan.com>
Signed-off-by: Cyril Brulebois <kibi@debian.org>
Reviewed-By: Matthias Hopf <mhopf@suse.de>
Signed-off-by: Keith Packard <keithp@keithp.com>

hw/xfree86/modes/xf86Cursors.c

@@ -609,7 +609,7 @@ xf86_reload_cursors (ScreenPtr screen)
     cursor_screen_priv = dixLookupPrivate(&screen->devPrivates,
 					  xf86CursorScreenKey);
     /* return if HW cursor is inactive, to avoid displaying two cursors */
-    if (!cursor_screen_priv->isUp)
+    if (!cursor_screen_priv || !cursor_screen_priv->isUp)
 	return;
 
     scrn = xf86Screens[screen->myNum];