BUG/MEDIUM: mux_pt: Don't destroy the connection if we have a stream attached.

BUG/MEDIUM: mux_pt: Don't destroy the connection if we have a stream attached.

There's a small window where the mux_pt tasklet may be woken up, and thus
mux_pt_io_cb() get scheduled, and then the connection is attached to a new
stream. If this happen, don't do anything, and just let the stream know
by calling its wake method. If the connection had an error, the stream should
take care of destroying it by calling the detach method.

This should be backported to 2.0 and 1.9.