shell: fix handle_lock_surface_destroy()

A copy & paste bug, that resulted setting to NULL something else than
shell->lock_surface when that surface was destroyed.

The symptom: let compositor lock down, unlock it, let it lock down
again, and the unlock dialog is never requested again. This bug was
triggered by the previous fix "shell: fix compositor wakeup while
locked".

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

compositor/shell.c

@@ -834,7 +834,7 @@ handle_lock_surface_destroy(struct wl_listener *listener,
 			    struct wl_resource *resource, uint32_t time)
 {
 	struct wl_shell *shell =
-		container_of(listener, struct wl_shell, panel_listener);
+		container_of(listener, struct wl_shell, lock_surface_listener);
 
 	fprintf(stderr, "lock surface gone\n");
 	shell->lock_surface = NULL;