compositor: fix two crashs on surface_attach() with null buffer

The condition to return from surface_attach with a null buffer involves
es->output being non-null. However if a surface was just created this
field would be null and an attach of a null buffer would cause the
compositor to crash.

The other crash happened if surface_attach was called twice with a
null buffer after a valid buffer was attached to the surface. Since
es->buffer was not being set to NULL, surface_attach() would call
wl_list_remove(&es->buffer_destroy_listener.link) twice for the same
surface.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>

src/compositor.c

@@ -1151,6 +1151,9 @@ surface_attach(struct wl_client *client,
 	struct weston_shell *shell = es->compositor->shell;
 	struct wl_buffer *buffer;
 
+	if (!buffer_resource && !es->output)
+		return;
+
 	weston_surface_damage_below(es);
 
 	if (es->buffer) {
@@ -1162,6 +1165,7 @@ surface_attach(struct wl_client *client,
 		wl_list_remove(&es->link);
 		es->visual = WESTON_NONE_VISUAL;
 		es->output = NULL;
+		es->buffer = NULL;
 		return;
 	}