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: NBaozeng Ding <sploving1@gmail.com>
Signed-off-by: NRichard Alpe <richard.alpe@ericsson.com>
Acked-by: NJon Maloy <jon.maloy@ericsson.cm>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>