wayland/surface: Check for surface role in meta_wayland_surface_get_window()

The function can get called without valid surface role, e.g. from
`zwp_xwayland_keyboard_grab_manager_grab()`.

Debugged by @piegamesde

Fixes https://gitlab.gnome.org/GNOME/mutter/-/issues/1147

src/wayland/meta-wayland-surface.c

@@ -1745,6 +1745,9 @@ meta_wayland_surface_role_get_window (MetaWaylandSurfaceRole *surface_role)
 MetaWindow *
 meta_wayland_surface_get_window (MetaWaylandSurface *surface)
 {
+  if (!surface->role)
+    return NULL;
+
   return meta_wayland_surface_role_get_window (surface->role);
 }