if received undefined value, length output warninig.

so, this change is inhibit warning.
JSON::XS falls on blessed value. so, if it falls, blame to caller.
change to $Carp::CarpLevel.

呼び出し元（Ze を使う側）に責任を押し付けたいから。
Merge pull request #1 from vkgtaro/master

呼び出し元に責任を押し付けたい