Segmentation fault: 11 error on OSX in release mode

Works in debug mode but fails in release mode.

OSX 10.11.5

Tested with:

- rustc 1.26.2 (594fb253c 2018-06-01)
- rustc 1.28.0-nightly (967c1f3be 2018-06-15)

```rust
extern crate regex;

use regex::Regex;

fn main() {
    let regex = Regex::new("^1$").unwrap();
    println!("{}", regex.is_match("1"));
}
```

