can: j1939: j1939_sk_setsockopt(): prevent allocation of j1939 filter for optlen == 0

If optval != NULL and optlen == 0 are specified for SO_J1939_FILTER in
j1939_sk_setsockopt(), memdup_sockptr() will return ZERO_PTR for 0
size allocation. The new filter will be mistakenly assigned ZERO_PTR.
This patch checks for optlen != 0 and filter will be assigned NULL in
case of optlen == 0.

Fixes: 9d71dd0c ("can: add support of SAE J1939 protocol")
Link: https://lore.kernel.org/r/20210620123842.117975-1-nslusarek@gmx.netSigned-off-by: NNorbert Slusarek <nslusarek@gmx.net>
Acked-by: NOleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

net/can/j1939/socket.c

@@ -676,7 +676,7 @@ static int j1939_sk_setsockopt(struct socket *sock, int level, int optname,
 
 	switch (optname) {
 	case SO_J1939_FILTER:
-		if (!sockptr_is_null(optval)) {
+		if (!sockptr_is_null(optval) && optlen != 0) {
 			struct j1939_filter *f;
 			int c;