[mutter] window-x11: Fix stage window check when running under non-X11 backends

[Jasper St. Pierre <jstpierre src gnome org>]

commit b8c13cc426314048aa2cefb3ed082f798683dd33
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Fri Aug 15 12:37:15 2014 -0400

    window-x11: Fix stage window check when running under non-X11 backends
    
    This code also runs when under the native backend for XWayland, so we
    can't crash on an invalid cast there.

 src/x11/window-x11.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/src/x11/window-x11.c b/src/x11/window-x11.c
index ebb6ec2..d4b9683 100644
--- a/src/x11/window-x11.c
+++ b/src/x11/window-x11.c
@@ -2803,9 +2803,13 @@ is_our_xwindow (MetaDisplay       *display,
     return TRUE;
 
   {
-    MetaBackendX11 *backend = META_BACKEND_X11 (meta_get_backend ());
-    if (xwindow == meta_backend_x11_get_xwindow (backend))
-      return TRUE;
+    MetaBackend *backend = meta_get_backend ();
+
+    if (META_IS_BACKEND_X11 (backend))
+      {
+        if (xwindow == meta_backend_x11_get_xwindow (META_BACKEND_X11 (backend)))
+          return TRUE;
+      }
   }
 
   /* Any windows created via meta_create_offscreen_window */