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>