in_forward: fix checking return value of cmt_decode_msgpack_create(#8000)

[nokute78]

cmt_encode_msgpack_create can return positive error code like CMT_DECODE_MSGPACK_INVALID_ARGUMENT_ERROR.
We should check if the return value is CMT_DECODE_MSGPACK_SUCCESS or not.

#8000 can be caused if the API returns positive error and cmt is invalid pointer.

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