desktop-shell: NULL check whether a popup parent is a shell surface

get_shell_surface(parent) may return NULL if the client passed a
unassigned wl_surface or a wl_surface with a non-shell surface role
(such as cursor role).

https://bugs.freedesktop.org/show_bug.cgi?id=92316

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Tested-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>

desktop-shell/shell.c

@@ -4279,8 +4279,9 @@ xdg_get_xdg_popup(struct wl_client *client,
 	 * top level or not. */
 
 	parent_shsurf = get_shell_surface(parent);
-	if (!shell_surface_is_xdg_popup(parent_shsurf) &&
-	    !shell_surface_is_xdg_surface(parent_shsurf)) {
+	if (!parent_shsurf ||
+	    (!shell_surface_is_xdg_popup(parent_shsurf) &&
+	     !shell_surface_is_xdg_surface(parent_shsurf))) {
 		wl_resource_post_error(resource,
 				       XDG_SHELL_ERROR_INVALID_POPUP_PARENT,
 				       "xdg_popup parent was invalid");