tor/changes/bug40644

  o Minor bugfixes (congestion control):
    - Add a check for an integer underflow condition that might
      happen in cases where the system clock is stopped, the
      ORconn is blocked, and the endpoint sends more than a
      congestion window worth of non-data control cells at once.
      This would cause a large congestion window to be calculated
      instead of a small one. No security impact. Fixes bug 40644;
      bugfix on 0.4.7.5-alpha.