winex11: Ignore color key and exposures when using the null surface.

dlls/winex11.drv/bitblt.c

@@ -2059,6 +2059,8 @@ void set_surface_color_key( struct window_surface *window_surface, COLORREF colo
     struct x11drv_window_surface *surface = get_x11_surface( window_surface );
     COLORREF prev;
 
+    if (window_surface->funcs != &x11drv_surface_funcs) return;  /* we may get the null surface */
+
     window_surface->funcs->lock( window_surface );
     prev = surface->color_key;
     set_color_key( surface, color_key );
@@ -2074,6 +2076,8 @@ HRGN expose_surface( struct window_surface *window_surface, const RECT *rect )
     struct x11drv_window_surface *surface = get_x11_surface( window_surface );
     HRGN region = 0;
 
+    if (window_surface->funcs != &x11drv_surface_funcs) return 0;  /* we may get the null surface */
+
     window_surface->funcs->lock( window_surface );
     add_bounds_rect( &surface->bounds, rect );
     if (surface->region)