Fatal error: String index is out of bounds

Hi,

The following code crashes on the last line with a fatal error : _String index is out of bounds_ instead of throwing an error

```
let scanner = StringScanner("1.0")
try scanner.skip(charactersIn: .whitespaces)
let red = try scanner.scanFloat()
try scanner.skip(charactersIn: .whitespaces)
let green = try scanner.scanFloat()
```