o Minor bugfixes (spec conformance):
    - Reject EXTEND cells sent to nonexistent streams. According to the
      spec, an EXTEND cell sent to _any_ nonzero stream ID is invalid, but
      we were only checking for stream IDs that were currenty in use.
      Found while hunting for more instances of bug 6271. Bugfix on
      0.0.2pre8, which introduced incremental circuit construction.