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: 9d71dd0c70 ("can: add support of SAE J1939 protocol")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>

net/can/j1939/main.c

@@ -207,6 +207,9 @@ static inline struct j1939_priv *j1939_ndev_to_priv(struct net_device *ndev)
 {
 	struct can_ml_priv *can_ml_priv = ndev->ml_priv;
 
+	if (!can_ml_priv)
+		return NULL;
+
 	return can_ml_priv->j1939_priv;
 }