blockchain: check total difficulty only when having same justified bl…

…ock number (axieinfinity#321)

In fork choice, we only need to compare the difficulty of forks when the
justified block numbers are the same in those forks. Otherwise, we choose the
fork with higher justified block number. This commit fixes a bug in current fork
choice to make it follow the correct behavior.

core/blockchain.go

@@ -1331,6 +1331,8 @@ func (bc *BlockChain) reorgNeeded(localBlock *types.Block, localTd *big.Int, ext
 
 		if externJustifiedBlockNumber > localJustifiedBlockNumber {
 			return true
+		} else if externJustifiedBlockNumber < localJustifiedBlockNumber {
+			return false
 		}
 	}