tor/changes/bug6252_again

  o Security fixes:
    - Tear down the circuit if we get an unexpected SENDME cell. Clients
      could use this trick to make their circuits receive cells faster
      than our flow control would have allowed, or to gum up the network,
      or possibly to do targeted memory denial-of-service attacks on
      entry nodes. Fixes bug 6252. Bugfix on the 54th commit on Tor --
      from July 2002, before the release of Tor 0.0.0.  We had committed
      this patch previously, but we had to revert it because of bug 6271.
      Now that 6271 is fixed, this appears to work.