Update cwt/cose_message.py

Co-authored-by: Ajitomi Daisuke <dajiaji@gmail.com>

Author: kentakayama

cwt/cose_message.py

@@ -337,7 +337,7 @@ def attach_payload(self: Self, payload: bytes) -> COSEMessage:
             ValueError: The payload already exist.
         """
 
-        if self._payload is not None:
+        if self._msg[2] is not None:
             raise ValueError("The payload already exist.")
         self._payload = payload