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>