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.net


Signed-off-by: Norbert Slusarek <nslusarek@gmx.net>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>