bluetooth: host: Fix NULL pointer dereference in bt_conn_auth_cb_overlay

There is a check in bt_conn_auth_cb_overlay function which validates
if content of the callback structure is correct, but there is no
NULL-check on the structure pointer itself, which could result in
NULL pointer dereference.

It should be possible to set the callback structure pointer to `NULL`
using bt_conn_auth_cb_overlay function if the application requires
ex. Just Works pairing for one Bluetooth identity and global
callbacks are configured for advanced pairing scheme (like Passkey
Display) for other Bluetooth identity.

Signed-off-by: Mateusz Kapala <mateusz.kapala@nordicsemi.no>