files: Correctly handle tasks with no open file descriptors

Do not fail, if task doesn't have open files. This fixes
zdtm/static/fd test, which is broken now:

(00.004411) Error (cr-dump.c:1312): Dump files (pid: 25) failed with -1
(00.004548) Error (cr-dump.c:1614): Dumping FAILED.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/files.c

@@ -481,6 +481,7 @@ int dump_task_files_seized(struct parasite_ctl *ctl, struct pstree_item *item,
 	if (!img)
 		goto err;
 
+	ret = 0; /* Don't fail if nr_fds == 0 */
 	for (off = 0; off < dfds->nr_fds; off += nr_fds) {
 		if (nr_fds + off > dfds->nr_fds)
 			nr_fds = dfds->nr_fds - off;