wayland: don't free surfaces that have a window associated

After a MetaWaylandSurface is associated with a MetaWindow, it
should be freed only when the MetaWindow is unmanaged. For wayland
clients, the window is unmanaged when the resource is destroyed,
but for X11 clients we want to wait for the unmap event.

https://bugzilla.gnome.org/show_bug.cgi?id=705917

src/wayland/meta-wayland.c

@@ -537,7 +537,7 @@ meta_wayland_surface_resource_destroy_cb (struct wl_resource *resource)
 
 	  meta_window_unmanage (surface->window, timestamp);
 	}
-      else
+      else if (!surface->window)
 	meta_wayland_surface_free (surface);
     }
 }