message: validation: assert that pindexPrev is non-null when required
diff: @@ -3267,6 +3267,7 @@ static bool ContextualCheckBlock(const CBlock& block, CValidationState& state, c
  // Start enforcing BIP113 (Median Time Past) using versionbits logic.
  int nLockTimeFlags = 0;
  if (VersionBitsState(pindexPrev, consensusParams, Consensus::DEPLOYMENT_CSV, versionbitscache) == ThresholdState::ACTIVE) {
  + assert(pindexPrev != nullptr);
  nLockTimeFlags |= LOCKTIME_MEDIAN_TIME_PAST;
  }
  