message: _idx checks were already made in _Contains, just fetch the index
diff: @@ -50,7 +50,7 @@ CBlockLocator CChain::GetLocator(const CBlockIndex *pindex) const
  if (_Contains(pindex))
  {
  // Use O(1) CChain index if possible.
  - pindex = _idx(nHeight);
  + pindex = vChain[nHeight];
  }
  else
  {
  