core: initialize current block/fastblock atomics to nil, fix ethereum…

…#19286 (ethereum#19352)

core/blockchain.go

@@ -232,10 +232,16 @@ func NewBlockChain(db ethdb.Database, cacheConfig *CacheConfig, chainConfig *par
 	if bc.genesisBlock == nil {
 		return nil, ErrNoGenesis
 	}
+
+	var nilBlock *types.Block
+	bc.currentBlock.Store(nilBlock)
+	bc.currentFastBlock.Store(nilBlock)
+
 	// Initialize the chain with ancient data if it isn't empty.
 	if bc.empty() {
 		rawdb.InitDatabaseFromFreezer(bc.db)
 	}
+
 	if err := bc.loadLastState(); err != nil {
 		return nil, err
 	}