Bluetooth: Fix possible NULL pointer dereference

Checking conn->pending_sec_level if there is no connection leads to potential
null pointer dereference. Don't process pin_code_request_event at all if no
connection exists.

Signed-off-by: Waldemar Rymarkiewicz <waldemar.rymarkiewicz@gmail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>