netfilter: ipset: fix crash if IPSET_CMD_NONE command is sent

This patch fixes a crash if that ipset command is sent over nfnetlink.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Acked-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>