les: fix les/1 CHT compatibility issue (ethereum#15692)

Author: zsfelfoldi

les/handler.go

@@ -846,8 +846,8 @@ func (pm *ProtocolManager) handleMsg(p *peer) error {
 			}
 
 			if header := pm.blockchain.GetHeaderByNumber(req.BlockNum); header != nil {
-				sectionHead := core.GetCanonicalHash(pm.chainDb, (req.ChtNum+1)*light.ChtV1Frequency-1)
-				if root := light.GetChtRoot(pm.chainDb, req.ChtNum, sectionHead); root != (common.Hash{}) {
+				sectionHead := core.GetCanonicalHash(pm.chainDb, req.ChtNum*light.ChtV1Frequency-1)
+				if root := light.GetChtRoot(pm.chainDb, req.ChtNum-1, sectionHead); root != (common.Hash{}) {
 					if tr, _ := trie.New(root, trieDb); tr != nil {
 						var encNumber [8]byte
 						binary.BigEndian.PutUint64(encNumber[:], req.BlockNum)