uffd: don't close the same descriptor twice

>>> >>>     CID 161322:  API usage errors  (USE_AFTER_FREE)
>>> >>>     Calling "close(int)" closes handle "client" which has already been closed.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>