core: reorg logs crashed, add a check for corner cases

core/blockchain.go

@@ -1145,13 +1145,16 @@ func (self *BlockChain) reorg(oldBlock, newBlock *types.Block) error {
 		}
 	}
 	// Ensure the user sees large reorgs
+	if len(oldChain) > 0 && len(newChain) > 0 {
 		logFn := log.Debug
 		if len(oldChain) > 63 {
 			logFn = log.Warn
 		}
 		logFn("Chain split detected", "number", commonBlock.Number(), "hash", commonBlock.Hash(),
 			"drop", len(oldChain), "dropfrom", oldChain[0].Hash(), "add", len(newChain), "addfrom", newChain[0].Hash())
-
+	} else {
+		log.Error("Impossible reorg, please file an issue", "oldnum", oldBlock.Number(), "oldhash", oldBlock.Hash(), "newnum", newBlock.Number(), "newhash", newBlock.Hash())
+	}
 	var addedTxs types.Transactions
 	// insert blocks. Order does not matter. Last block will be written in ImportChain itself which creates the new head properly
 	for _, block := range newChain {