Bluetooth: Host: Abort pairing if disconnected

If disconnection has been triggered in between the security update and
the call to `smp_pairing_complete` we need to abort the pairing.

The disconnection may have been triggered by `bt_unpair`, in that case
the keys will have been erased and it will lead to an assertion to
continue as if nothing happened.

To resolve this issue, at the beginning of `smp_pairing_complete` the
`status` is set to `BT_SMP_ERR_UNSPECIFIED` if there is no connection.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>