Fix bug in PAN code where no new PAN connections can be made.

If you call PAN_connect() followed quickly by PAN_disconnect(),
you won't be able to establish any additional PANU connections.
This happens because the connection never has a chance to make
it into PAN_STATE_CONNECTED and num_conns only gets decremented
if you disconnect in the connected state. As a result, subsequent
calls to connect will fail because the stack still believes there
to be an existing connection.

Change-Id: Id24873828ca4f517496cb19a44993d0e8e837a1e