message: Reject blocks with null pindexPrev
  During validation, there should be no instances of a block reaching
  `CheckWork` that has a NULL pindexPrev, however if such an instance
  DOES occur, reject the block.
diff: @@ -3395,14 +3395,19 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
  bool CheckWork(const CBlock block, CBlockIndex* const pindexPrev)
  {
  + if (pindexPrev == NULL)
  + return error("%s : null pindexPrev for block %s", __func__, block.GetHash().ToString().c_str());
  +
  unsigned int nBitsRequired = GetNextWorkRequired(pindexPrev, &block);
  - if (block.IsProofOfWork() && (pindexPrev == NULL || pindexPrev->nHeight + 1 <= 68589)) {
  + if (block.IsProofOfWork() && (pindexPrev->nHeight + 1 <= 68589)) {
  double n1 = ConvertBitsToDouble(block.nBits);
  double n2 = ConvertBitsToDouble(nBitsRequired);
  if (abs(n1 - n2) > n1 * 0.5)
  return error("%s : incorrect proof of work (DGW pre-fork) - %f %f %f at %d", __func__, abs(n1 - n2), n1, n2, pindexPrev->nHeight + 1);
  +
  + return true;
  }
  if (block.nBits != nBitsRequired)
  