SIGSEGV on MacOS

I'm seeing crashes on MacOS Catalina (10.15.3) with Rust 1.43.1. This happens in the [original clipboard crate](https://github.com/aweinstock314/rust-clipboard) as well, but since that seems unmaintained I'm reporting it here.

You can reproduce the crash by running `cargo test` in my [fork](https://github.com/RoyJacobs/copypasta/tree/osx-failure).

You may need to run it a few times, but you will get different errors such as:
```
failing-517cc89e9cb2a459(35714,0x70000ebe5000) malloc: *** error for object 0x7f8eb170a6e0: pointer being freed was not allocated
failing-517cc89e9cb2a459(35714,0x70000ebe5000) malloc: *** set a breakpoint in malloc_error_break to debug
```

Or:
```
failing-517cc89e9cb2a459(35969,0x700000ae7000) malloc: Double free of object 0x7fe935c10a50
failing-517cc89e9cb2a459(35969,0x700000ae7000) malloc: *** set a breakpoint in malloc_error_break to debug
```

I'm unfortunately no MacOS developer so I have no idea what causes this, but it only seems to occur when I'm accessing the clipboard from both my test code and my regular (under test) code, like in `some_other_fn` in the code below. Moving the `ClipboardContext` to a shared static using something like `lazy_static!` does not help.

The test is fairly simple:

```
use copypasta::{ClipboardContext, ClipboardProvider};

fn some_other_fn() {
let mut ctx = ClipboardContext::new().unwrap();
ctx.get_contents().unwrap();
}

#[cfg(test)]
mod tests {
use super::*;

fn clear_clipboard() {
let mut clipboard: ClipboardContext = ClipboardContext::new().unwrap();
clipboard.set_contents("".into()).unwrap();
}

#[test]
fn foo() {
clear_clipboard();

let mut ctx = ClipboardContext::new().unwrap();
ctx.set_contents("Dummy".into()).unwrap();
ctx.get_contents().unwrap();

some_other_fn();
}

#[test]
fn bar() {
clear_clipboard();

let mut ctx = ClipboardContext::new().unwrap();
ctx.set_contents("Dummy".into()).unwrap();
ctx.get_contents().unwrap();

some_other_fn();
}
}
```