core: prevent nil pointer exception

core/blockchain.go

@@ -2059,7 +2059,9 @@ func (bc *BlockChain) reorg(oldBlock, newBlock *types.Block) error {
 	}
 	// Insert the new chain(except the head block(reverse order)),
 	// taking care of the proper incremental order.
-	log.Warn("Long reorg", "newchain length", len(newChain), "from", newChain[0].number, "to", newChain[len(newChain)-1].number)
+	if len(newChain) > 0 {
+		log.Warn("Long reorg", "newchain length", len(newChain), "from", newChain[0].number, "to", newChain[len(newChain)-1].number)
+	}
 	for i := len(newChain) - 1; i >= 1; i-- {
 		// Insert the block in the canonical way, re-writing history
 		block := bc.GetBlock(newChain[i].hash, newChain[i].number)