can: j1939: main: j1939_ndev_to_priv(): avoid crash if can_ml_priv is NULL

This patch avoids a NULL pointer deref crash if ndev->ml_priv is NULL.

Reported-by:  <syzbot+95c8e0d9dffde15b6c5c@syzkaller.appspotmail.com>
Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>