Fenwick Tree's select method gives incorrect answer

Sometimes the method won't return the correct value.

How to reproduce:

```cpp
FenwickTree ft(10);
for (int i = 1; i <= 10; i++){
    ft.update(i,1);
}
for (int i = 1; i <= 10; i++){
    cout << ft.select(i) << "\n";
}
```
Expected output:
```
1
2
3
4
5
6
7
8
9
10
```
Obtained result:
```
1
2
3
4
5
6
7
8
16
16
```
Code execution: https://ideone.com/k5IMXN
