message: Allow CheckWork() to reindex the genesis block
  During a reindex, the genesis block passes through CheckWork. Since
  the genesis block has no previous block hash, we must guard the check
  to avoid a segfault.
diff: @@ -3552,7 +3552,7 @@ bool CheckWork(const CBlock block, CBlockIndex * const pindexPrev)
  else if (block.IsProofOfWork())
  {
  // Check proof of work (Here for the architecture issues with DGW v1 and v2)
  - if(pindexPrev->nHeight + 1 <= 68589)
  + if(pindexPrev == NULL || pindexPrev->nHeight + 1 <= 68589)
  {
  unsigned int nBitsNext = GetNextWorkRequired(pindexPrev, &block, false);
  double n1 = ConvertBitsToDouble(block.nBits);
  