Bluetooth: Fix NULL pointer dereference in smp_conn_security

The l2cap_conn->smp pointer may be NULL for various valid reasons where SMP has
failed to initialize properly. One such scenario is when crypto support is
missing, another when the adapter has been powered on through a legacy method.
The smp_conn_security() function should have the appropriate check for this
situation to avoid NULL pointer dereferences.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 4.0+