Make json_object::clone Exception-Safe.
Make json_object::clone Exception-Safe.

https://github.com/takei-yuya/pficommon/commit/2cc7cbcb0cd7aafef44be992934e89c535e03081#commitcomment-1175857

takei-yuya/pficommon,retrieva/pficommon,retrieva/pficommon,takei-yuya/pficommon,takei-yuya/pficommon,pfi/pficommon,pfi/pficommon,pfi/pficommon,retrieva/pficommon,retrieva/pficommon,takei-yuya/pficommon,pfi/pficommon,retrieva/pficommon