unix: don't leak a file descriptor

1359            if (prep_unix_sk_cwd(ui, &cwd_fd, &root_fd))
>>> >>>     CID 175319:  Resource leaks  (RESOURCE_LEAK)
>>> >>>     Handle variable cwd_fd going out of scope leaks the handle.

travis-ci: success for unix: don't leak a file descriptor
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>