fuzzing report

First: is this a bug report? A suggestion? Or asking for help?

Just some information, and a small suggestion.

# Problem description

I wrote a small setup to fuzz the display of untrusted strings with cursive: https://github.com/alexanderkjall/cursive/commit/fada64679f1c37eec59da9662404165d2602e4a2

### default ncurses backend

```
thread '' panicked at 'called `Result::unwrap()` on an `Err` value: NulError(0, [0])', /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/ncurses-5.99.0/src/lib.rs:65:29
```

### crossterm backend

No issue found after half an hour of fuzzing.

### pancurses-backend
```
thread '' panicked at 'called `Result::unwrap()` on an `Err` value: NulError(0, [0])', /home/capitol/.cargo/registry/src/github.com-1ecc6299db9ec823/pancurses-0.16.1/src/window.rs:392:47
```

### termion-backend

```
thread '' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 24, kind: Other, message: "Too many open files" }', /home/capitol/projects/cursive/cursive/src/cursive_runnable.rs:65:64
```

This panic is in my fuzzing code, so it might be a false positive, but might also be some sort of resource leakage.

### blt-backend

no fuzzing done, had a hard time to get this to work on ubuntu and my arch system isn't available right now.

## Suggestions

I have not gone over and replicated the issues in each of the underlying libraries and reported issues there yet.

Looking at the open issues in the ncurses library, it looks like there is a lot of security issues open that hasn't been addressed yet: https://github.com/jeaye/ncurses-rs/issues

I would recommend to either switch the default backend to a more safe one, or at least have a warning note about it in the documentation, as cursive lables itself `It is designed to be safe and easy to use`.

# Environment

* Operating system: linux (ubuntu 20.04)
* Backend used: all of them
* Cursive version: from git