Improve descriptor checks in the new guard algorithm.

- Make sure we check at least two guards for descriptor before making
  circuits. We typically use the first primary guard for circuits, but
  it can also happen that we use the second primary guard (e.g. if we
  pick our first primary guard as an exit), so we should make sure we
  have descriptors for both of them.

- Remove BUG() from the guard_has_descriptor() check since we now know
  that this can happen in rare but legitimate situations as well, and we
  should just move to the next guard in that case.