message: Fix Thinblock Missing transactiosn log message
  We were getting a -1 for number of txns missing from the thinblock
  caused by clearing the thindata before printing out the message.
  (Backport to `release` branch of PR 570)
diff: @@ -184,15 +184,15 @@ bool CThinBlock::process(CNode *pfrom, int nSizeThinBlock, string strCommand)
  {
  // This marks the end of the transactions we've received. If we get this and we have NOT been able to
  // finish reassembling the block, we need to re-request the full regular block
  + LogPrint("thin", "Missing %d Thinblock transactions, re-requesting a regular block\n",
  + pfrom->thinBlockWaitingForTxns);
  + thindata.UpdateInBoundReRequestedTx(pfrom->thinBlockWaitingForTxns);
  thindata.ClearThinBlockData(pfrom);
  vector<CInv> vGetData;
  vGetData.push_back(CInv(MSG_BLOCK, header.GetHash()));
  pfrom->PushMessage(NetMsgType::GETDATA, vGetData);
  setPreVerifiedTxHash.clear(); // Xpress Validation - clear the set since we do not do XVal on regular blocks
  - LogPrint("thin", "Missing %d Thinblock transactions, re-requesting a regular block\n",
  - pfrom->thinBlockWaitingForTxns);
  - thindata.UpdateInBoundReRequestedTx(pfrom->thinBlockWaitingForTxns);
  }
  return true;
  