fix possible shutdown assertion with -reindex-shutdown

Credit @eklitzke for reproducing.

Github-Pull: #12349
Rebased-From: ceaefdd5f362537a1908d0095059e4be788f3dee
Tree-SHA512: bdc614d3c3fba23147be9528c581e25bbf1f0c359b525b4a05472ab42484724a8b34c8b3ed151f3ff23e48235e972950f9daa155d9ca3c4a9de6d61bf0591b4b

src/validation.cpp

@@ -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