Illegal division by 0

1. Slow down the program to 0 WPM by pressing [.
2. Pause using space.
3. Unpause using space.
4. Get error:

```
Illegal division by zero at /nix/store/hm9q7cmvf849qcp4kh0hf4xv42g3g3j4-speedread-unstable-2016-09-21/bin/speedread line 138, <> line 7.
```