fix array index crash

fix array index crash

Summary:
I didn't realize Rust does runtime check for over flow, so the the trick won't work. In the previous implementation, I tried to depend on the "wrap around" behavior of over flowed subtraction operation.

Given usize is return by `len()` and `i >= 0` is always `true`, let `i` be the current length seems the easiest implementation.

```
fn rfind(s: &[u8], needle: u8) -> Option<usize> {
    let mut i = s.len();
    while i > 0 {
        i -= 1;
        if s[i] == needle {
            return Some(i);
        }
    }
    None
}
```

Reviewed By: losvald

Differential Revision: D18219014

fbshipit-source-id: 0d0ce8ea744e2ed30c2702c40a67ad7fd6270e2f