Without this fix the panic looks like the following:

```rust
---- parser::tests::lexer::string_errors stdout ----
thread 'parser::tests::lexer::string_errors' panicked at 'byte index 4 is not a char boundary; it is inside 'ɠ' (bytes 3..5) of `"\uɠ^A`', src/libcore/str/mod.rs:2219:5
```

This was found via fuzzing with `cargo-fuzz`.