dix: extra NULL protection in UnmapSubwindows()

Even though it probably won't be hit ever, it's still better to be
really sure instead of some remote chance for hard segfault.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/window.c

@@ -2915,7 +2915,7 @@ UnmapSubwindows(WindowPtr pWin)
                 UnrealizeTree(pChild, FALSE);
         }
     }
-    if (wasViewable && anyMarked) {
+    if (wasViewable && anyMarked && pLayerWin) {
         if (pLayerWin->parent == pWin)
             (*pScreen->MarkWindow) (pWin);
         else {