sk-unix: don't add a socket in a second time

A socket can be added in a list two times, if two other sockets
are connected to it.

This patch fixes a problem:
    Error (sk-unix.c:395): BUG at sk-unix.c:395
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

sk-unix.c

@@ -163,7 +163,7 @@ static int dump_one_unix_fd(int lfd, u32 id, const struct fd_parms *p)
 			 * It can be external socket, so we defer dumping
 			 * until all sockets the program owns are processed.
 			 */
-			if (!peer->sd.already_dumped) {
+			if (!peer->sd.already_dumped && !list_empty(&peer->list)) {
 				show_one_unix("Add a peer", peer);
 				list_add_tail(&peer->list, &unix_sockets);
 			}