set_status_to_connected: do nothing if we already disconnected
set_status_to_connected: do nothing if we already disconnected

Now that we're making more use of GAsyncResult, we can easily get into
this situation:

* all but one of the preconditions for being CONNECTED have happened;
  the remaining one uses GAsyncResult (currently that can only be
  conn_presence_set_initial_presence_async)

* conn_presence_set_initial_presence succeeds, and schedules a call
  to its callback in an idle; we are now ready to be CONNECTED, but because
  of GAsyncResult calling conventions, we won't call the callback right now

* someone calls Disconnect(), and we do so, synchronously; we are now
  DISCONNECTED

* the idle goes off and connection_initial_presence_cb is called,
  with success, while DISCONNECTED!

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37078
Reviewed-by: Vivek Dasmohapatra <be459340ddb9c8286fc08bebdce70ca287e03a1a@collabora.co.uk>

jku/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble