[scn] fix failure of mittest after refresh feature scn

src/sql/engine/px/datahub/components/ob_dh_sample.cpp

@@ -595,6 +595,7 @@ int ObDynamicSamplePieceMsgCtx::on_message(
         LOG_WARN("failed to wait response", K(ret));
       }
     }
+    IGNORE_RETURN destroy();
   }
   return ret;
 }