Avoid crashing on busy/NEWNYM+hidden service clients

When we ran out of intro points for a hidden service (which could
happen on a newnym), we would change the connection's state back to
"waiting for hidden service descriptor."  But this would make an
assertion fail if we went on to call circuit_get_open_circ_or_launch
again.

This fixes bug 16013; I believe the bug was introduced in
38be533c, where we made it possible for
circuit_get_open_circ_or_launch() to change the connection's state.