dix: don't try to create events if we don't have a screen. #16898

A NULL screen may happen during server shutdown, when the output has been shut
down but the devices still generate events.

X.Org Bug 16898 <http://bugs.freedesktop.org/show_bug.cgi?id=16898>
(cherry picked from commit 465a9bb96e)

dix/getevents.c

@@ -535,6 +535,8 @@ GetPointerEvents(xEvent *events, DeviceIntPtr pDev, int type, int buttons,
     ScreenPtr scr = miPointerGetScreen(pDev);
 
     /* Sanity checks. */
+    if (!scr) /* can happen during server shutdown */
+        return 0;
     if (type != MotionNotify && type != ButtonPress && type != ButtonRelease)
         return 0;