Bluetooth: Fix missing check for SMP session in smp_user_confirm_reply

The smp_user_confirm_reply() function is called whenever user space
sends a user confirmation reply mgmt command. In case of a misbehaving
user space, or if the SMP session was removed by the time the command
comes it is important that we return an appropriate error and do not try
to access the non-existent SMP context. This patch adds the appropriate
check for the HCI_CONN_LE_SMP_PEND flag before proceeding further.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>