Incorrect &str => f64 results

Perhaps an off-by-one-somewhere?

``` rust
fn main() {                                 
    let f: f64 = from_str("-3.14").unwrap();
    println!("{}", f);                      
}                                           
```

```
$ rustc foo.rs && ./foo
-13.14
```

cc @bjz, probably related to https://github.com/rust-lang/rust/pull/18536
