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>