libweston-desktop: don't crash when getting the pid for X clients

X client's don't have a wl_client associated with their
weston_desktop_client, so make sure to not use it.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
diff --git a/libweston-desktop/xwayland.c b/libweston-desktop/xwayland.c
index baacf7b..6b90c14 100644
--- a/libweston-desktop/xwayland.c
+++ b/libweston-desktop/xwayland.c
@@ -241,6 +241,8 @@
 	wl_resource_add_destroy_listener(wsurface->resource,
 					 &surface->resource_destroy_listener);
 
+	weston_desktop_surface_set_pid(surface->surface, 0);
+
 	return surface;
 }