unix: Fix nil dereference in find_queuer_for

When walking over unix sockets make sure the
queuer is present before accessing it.

https://jira.sw.ru/browse/PSBM-82796Reported-by: Vitaly Ostrosablin <vostrosablin@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>

criu/sk-unix.c

@@ -848,7 +848,7 @@ static struct unix_sk_info *find_queuer_for(int id)
 	struct unix_sk_info *ui;
 
 	list_for_each_entry(ui, &unix_sockets, list) {
-		if (ui->queuer->ue->id == id)
+		if (ui->queuer && ui->queuer->ue->id == id)
 			return ui;
 	}