If `$u->id` fails down below, then you're going to call `LJ::MemCache::delete( undef )`.

That should never happen (you explicitly verify that this is a user with the `want_user` call above), and `id` should never fail, but I guess the possibility kind of makes me ponder. I can't think of a way to fix that without more code, and I don't know if it's really worth it. Probably not.