sandboxfs: Fix resource leak

Now that we free resources in sandbox_fs_ls Coverity is letting us know
that in some cases we might leak.  So in case of error we should still
let os_dirent_free free anything that was allocated.

Fixes: 86167089 ("sandbox/fs: Free memory allocated by os_dirent_ls")
Reported-by: Coverity (CID: 153450)
Cc: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>