NETFILTER: ctnetlink: check for status attribute existence on conntrack creation

Check that status flags are available in the netlink message received
to create a new conntrack.

Fixes a crash in ctnetlink_create_conntrack when the CTA_STATUS attribute
is not present.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>