ceph_json: use different string constructor for parser buffer

Fixes: #6397
Previous implementation did not user the constructor with the length
param.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: don't append NULL char before json parsing

Parser handles that by itself.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #657 from ceph/wip-6397

Wip 6397

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>