Bluetooth: Fix potential NULL dereference in SMP channel setup

When the allocation of the L2CAP channel for the BR/EDR security manager
fails, then the smp variable might be NULL. In that case do not try to
free the non-existing crypto contexts

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>