message: Change the place where we check for nTxPersec < 0
  We need to do this check just after decrememnting nTxPerSec and
  not where wher increment it.  Otherwise we can end up not setting
  it's value to zero when we need to.
diff: @@ -1885,6 +1885,8 @@ void CTxMemPool::UpdateTransactionsPerSecondImpl(bool fAddTxn, const std::lock_g
  {
  nTxPerSec -= (nTxPerSec / TX_RATE_SMOOTHING_SEC) * nDeltaTime;
  nLastTime = nNow;
  + if (nTxPerSec < 0)
  + nTxPerSec = 0;
  }
  // Add the new tx to the rate
  @@ -1893,8 +1895,6 @@ void CTxMemPool::UpdateTransactionsPerSecondImpl(bool fAddTxn, const std::lock_g
  nCount++;
  // The amount that the new tx will add to the tx rate
  nTxPerSec += 1 / TX_RATE_SMOOTHING_SEC;
  - if (nTxPerSec < 0)
  - nTxPerSec = 0;
  }
  // Calculate the peak rate if we've gone more that 1 second beyond the last sample time.
  