stat: don't add duplicate clat entries for json

stat: don't add duplicate clat entries for json

For some reason the initial commit always filled all
entries with zeroes, when it should just stop printing.
I suspect this was carried over from the terse/csv output,
where we do the same. But there it's actually valid, as we
have to have a fixed number of fields. For json we both
don't need it, and it's causing us to create invalid json
when we have duplicate keys.

Fixes: https://github.com/axboe/fio/issues/511
Signed-off-by: Jens Axboe <axboe@kernel.dk>