str::get isn't overflow-checked with InclusiveRange

Repro: https://is.gd/lVsdTA
```rust
#![feature(inclusive_range_syntax)]
#![feature(str_checked_slicing)]

fn main(){
    println!("{:?}", &"hello".get(...usize::max_value()-1)); // None
    println!("{:?}", &"hello".get(...usize::max_value())); // Some("")
}
```

(The latter with `[]` correctly panics with "attempted to index str up to maximum usize".)