message: fix possible shutdown assertion with -reindex-shutdown
  Credit for reproducing.
  Github-Pull:
  Rebased-From:
  Tree-SHA512:
diff: @@ -2086,7 +2086,7 @@ bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &
  nLastWrite = nNow;
  }
  // Flush best chain related state. This can only be done if the blocks / block index write was also done.
  - if (fDoFullFlush) {
  + if (fDoFullFlush && !pcoinsTip->GetBestBlock().IsNull()) {
  // Typical Coin structures on disk are around 48 bytes in size.
  // Pushing a new one to the database can cause it to be written
  // twice (once in the log, and once in the tables). This is already
  