tipc: check nl sock before parsing nested attributes

tipc: check nl sock before parsing nested attributes

Make sure the socket for which the user is listing publication exists
before parsing the socket netlink attributes.

Prior to this patch a call without any socket caused a NULL pointer
dereference in tipc_nl_publ_dump().

Tested-and-reported-by: Baozeng Ding <sploving1@gmail.com>
Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.cm>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 45e093ae2830cd1264677d47ff9a95a71f5d9f9c)

Orabug: 23347071
CVE: CVE-2016-4951

Signed-off-by: Manjunath Govindashetty <manjunath.govindashetty@oracle.com>