message: Make null pointer check in ContextualCheckBlock explicit
  As per the logic in ContextualCheckBlock(..) in main.cpp is
  implictly not causing a null pointer dereference due to behavior
  of VersionBitsState(..) extraction function.
  This makes the check for pIndexPrev == NULL explicit and thus
  also avoids tripping static analysis tools.
  Closes
diff: @@ -4106,7 +4106,11 @@ bool ContextualCheckBlock(const CBlock &block, CValidationState &state, CBlockIn
  nLockTimeFlags |= LOCKTIME_MEDIAN_TIME_PAST;
  }
  - int64_t nLockTimeCutoff =
  + int64_t nLockTimeCutoff;
  + if (pindexPrev == NULL)
  + nLockTimeCutoff = block.GetBlockTime();
  + else
  + nLockTimeCutoff =
  (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST) ? pindexPrev->GetMedianTimePast() : block.GetBlockTime();
  // Check that all transactions are finalized
  