Merge bitcoin#12349: shutdown: fix crash on shutdown with reindex-cha…

…instate

ceaefdd fix possible shutdown assertion with -reindex-shutdown (Cory Fields)

Pull request description:

  Fixes the assertion error reported here: bitcoin#12349 (comment)

Tree-SHA512: db8e2a275f92a99df7f17852d00eba6df996e412aa3ed3853a9ea0a8cb9800760677532efd52f92abbf2cdcc4210957a87a5f919ac998d46c205365a7a7dffca

src/validation.cpp

@@ -2349,7 +2349,7 @@ 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