content_extra_field_weight() gives bogus answer when cache needs refreshing

While working on #361446: missing list of signup users inside node which makes nice use of the whole "extra fields" API in CCK to drag + drop signup info to control where on the node it is displayed, I ran into a bug in content_extra_field_weight(). It's a pretty small function:

function content_extra_field_weight($type_name, $pseudo_field_name) {
  $type = content_types($type_name);

  // If we don't have the requested item, this may be because the cached
  // information for 'extra' fields hasn't been refreshed yet.
  if (!isset($type['extra'][$pseudo_field_name])) {
    content_clear_type_cache();
  }

  if (isset($type['extra'][$pseudo_field_name])) {
    return $type['extra'][$pseudo_field_name]['weight'];
  }
}

The problem is that if we hit the case where we need to call content_clear_type_cache(), we leave $type with the same uninitialized values, test it again immediately thereafter, (no surprise) the weight is still undefined, so we return NULL.

The solution is trivial, after we clear the cache, we need to refresh $type...