[BUG] program crashing on particular strings

**Describe the bug**
i found this out while using [ncspot](https://github.com/hrkfdn/ncspot), here's the [original issue](https://github.com/hrkfdn/ncspot/issues/1647) raised there. Basically, for particular strings (containing hindi letters, for example) the program just crashes, with the panic message mentioning cursive_core

**To Reproduce**
this program reproduces the error:
```rust
use cursive::views::{Dialog, TextView};

fn main() {
// Creates the cursive root - required for every application.
let mut siv = cursive::default();

// Creates a dialog with a single "Quit" button
siv.add_layer(
Dialog::around(TextView::new("मराठी प्रार्थना - Shubham Karoti Kalyanam "))
.title("Cursive")
.button("Quit", |s| s.quit()),
);

// Starts the event loop.
siv.run();
}
```

**Expected behavior**
not to crash for any particular string

**Environment**
* arch linux