Panic in FluentResource::try_new

While fuzzing my own application, I found that the following line will panic:

```Rust
let _ = FluentResource::try_new(r#"a={"\ǔ"#.to_string());
```

with the message:

```
byte index 7 is not a char boundary; it is inside '\u{30c}' (bytes 6..8) of `a={"\ǔ `
```
