Fix possible cause of crash after metadata download

src/TrackerWatcherCommand.cc

@@ -155,6 +155,7 @@ bool UDPAnnRequest::success() const
 void UDPAnnRequest::stop(DownloadEngine* e)
 {
   if (req_) {
+    req_->user_data = nullptr;
     req_.reset();
   }
 }

src/UDPTrackerClient.cc

@@ -151,7 +151,6 @@ int UDPTrackerClient::receiveReply(std::shared_ptr<UDPTrackerRequest>& recvReq,
       logInvalidTransaction(remoteAddr, remotePort, action, transactionId);
       return -1;
     }
-    req->state = UDPT_STA_COMPLETE;
 
     auto connectionId = bittorrent::getLLIntParam(data, 8);
     A2_LOG_INFO(