consensus: fix delete the 1st validator from snapshot.recents list (#1344)

* consensus: fix delete the 1st validator from snapshot.recents list

* consensus:fix uint sub error with negative

consensus/parlia/parlia.go

@@ -1326,7 +1326,14 @@ func (p *Parlia) backOffTime(snap *Snapshot, header *types.Header, val common.Ad
 		if p.chainConfig.IsPlanck(header.Number) {
 			// reverse the key/value of snap.Recents to get recentsMap
 			recentsMap := make(map[common.Address]uint64, len(snap.Recents))
+			bound := uint64(0)
+			if n, limit := header.Number.Uint64(), uint64(len(validators)/2+1); n > limit {
+				bound = n - limit
+			}
 			for seen, recent := range snap.Recents {
+				if seen <= bound {
+					continue
+				}
 				recentsMap[recent] = seen
 			}