Bluetooth: Fix invalid memory access when there's no SMP channel

We only should try to free the SMP channel that was created if there
is a pending SMP session.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>