Bluetooth: host: Disconnect connection if no conn object is available

When receiving a connection complete event but no connection object are
available in the host something strange has happened. In this case
the controller might have a connection that cannot be controlled by the
application. It would then be sensible to disconnect this connection in
the controller.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>