message: Check previous block time only if previous block exists
diff: @@ -3636,7 +3636,7 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
  return state.DoS(100, false, REJECT_INVALID, "bad-diffbits", false, "incorrect proof of work");
  // Check timestamp against prev
  - if (block.IsProofOfStake() && block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
  + if (pindexPrev && block.IsProofOfStake() && block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
  return state.Invalid(false, REJECT_INVALID, "time-too-old", "block's timestamp is too early");
  // Check timestamp
  