Check for stream_id, not conn, on extend cells.

Extend cells aren't allowed to have a stream_id, but we were only
blocking them when they had a stream_id that corresponded to a
connection.  As far as I can tell, this change is harmless: it will
make some kinds of broken clients not work any more, but afaik nobody
actually make a client that was broken in that way.

Found while hunting for other places where we made the same mistake
as in 6271.

Bugfix on d7f50337 back from May 2003, which introduced
telescoping circuit construction into 0.0.2pre8.