tor/changes/bug21302

  o Minor bugfixes (hidden service):
    - Two possible underflow which would ultimately lead to creating a lot of
      introduction points circuits and closing them in a non stop loop. Fixes
      bug 21302; bugfix on 0.2.7.2-alpha.
    - Cleanup expiring intro point nodes if no circuit is associated to it
      anymore. It was causing, rarely, the service to not open enough
      introduction points circuit in the case we had dead expiring nodes.;
      bugfix on 0.2.7.2-alpha.