message: [DWG] Make sure OneBack exists
diff: @@ -315,8 +315,10 @@ unsigned int DarkGravityWave(const CBlockIndex* pindexLast, const Consensus::Par
  int64_t nOverdueTime = nTipToLastMatchingProof - nPowSpacing;
  if (!nTipToLastMatchingProof) {
  // Tip is our algo, so look one more back to see where we're at
  + if (pindexOneBack) {
  nOverdueTime = pindexLastMatchingProof->GetBlockTime() - pindexOneBack->GetBlockTime() - nPowSpacing;
  }
  + }
  if (fProofOfStake) {
  // The concept of Overdue is variable; as it's anywhere from 60 to 120s, based on how many blocks
  @@ -330,9 +332,8 @@ unsigned int DarkGravityWave(const CBlockIndex* pindexLast, const Consensus::Par
  int64_t nBlocksPoW = nBlocksMeasured - nCountBlocks;
  int64_t nBlocksIntended = nBlocksMeasured >> 1; // 50%
  int64_t nPosSpacing = nPowSpacing + (nPowSpacing * nBlocksPoW / nBlocksIntended);
  - if (nTipToLastMatchingProof) {
  nOverdueTime = nTipToLastMatchingProof - nPosSpacing;
  - } else {
  + if (!nTipToLastMatchingProof && pindexOneBack) {
  // Tip is our algo, so look one more back to see where we're at
  nOverdueTime = pindexLastMatchingProof->GetBlockTime() - pindexOneBack->GetBlockTime() - nPosSpacing;
  }
  