Rivendell bugfix

There used to be a bug that sometimes affected groups trying to enter
Rivendell. If character B followed character A, and both character tried
to "seek Rivendell", sometimes the following happened:

- A finds the way in
- B follows A inside Rivendell
- B immediately moves outside

From now on this shouldn't happen again; apart from this situation, entrance
into Rivendell should keep working exactly as before. If the problem happens
again, or in case of other oddness, please post a bug report.