Fixed https://github.com/msgpack/msgpack-c/pull/113
When the original PR was applied, the resource leak had been fixed, but a dual free problem had been happened.

When msgpack_unpacker_next returns MSGPACK_UNPACK_CONTINUE, msgpack_unpacked::zone is not replaced. Then mespack_zone_free is called twice with the same object. msgpack_unpacked_destroy frees msgpack::zone when it is not NULL and set it to NULL.

Also, fixed memory leak (msgpack_sbuffer) on the test code.