fix: NarrowMode getTotalNum is 0

so like 10.0.1-2.0.255

iterator.go

@@ -263,7 +263,7 @@ func (it *Iter) getTotalNum() uint64 {
 	case NarrowMode:
 		var ret = uint64(1)
 		for _, v := range it.classmate {
-			ret = ret * uint64(v.e-v.s+1)
+			ret = ret*uint64(v.e-v.s) + 1
 		}
 		return ret
 	}