tipc: do sanity check payload of a netlink message

When we initialize nlmsghdr with no payload inside tipc_nl_compat_dumpit()
the parsing function returns -EINVAL. We fix it by making the parsing call
conditional.
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Link: https://lore.kernel.org/r/20201215033151.76139-1-hoang.h.le@dektech.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/netlink_compat.c

@@ -213,12 +213,14 @@ static int __tipc_nl_compat_dumpit(struct tipc_nl_compat_cmd_dump *cmd,
 	}
 
 	info.attrs = attrbuf;
+
+	if (nlmsg_len(cb.nlh) > 0) {
 		err = nlmsg_parse_deprecated(cb.nlh, GENL_HDRLEN, attrbuf,
 					     tipc_genl_family.maxattr,
 					     tipc_genl_family.policy, NULL);
 		if (err)
 			goto err_out;
-
+	}
 	do {
 		int rem;