hw/xwin: Fix a potential crash in winRedrawScreenShadowDDNL()

Seen during shutdown when using '-fullscreen' and '-depth 8'

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/winshadddnl.c

@@ -986,6 +986,10 @@ winRedrawScreenShadowDDNL(ScreenPtr pScreen)
     RECT rcSrc, rcDest;
     POINT ptOrigin;
 
+    /* Return immediately if we didn't get needed surfaces */
+    if (!pScreenPriv->pddsPrimary4 || !pScreenPriv->pddsShadow4)
+        return;
+
     /* Get the origin of the window in the screen coords */
     ptOrigin.x = pScreenInfo->dwXOffset;
     ptOrigin.y = pScreenInfo->dwYOffset;