gnucompability(stty): it accepts 0 for ispeed but the gnu one rejects it

Hi, uutils mainteners

that is a compability issue with stty command as our test confirms

```css
relunsec@relunsec:~/software/coreutils/target/debug$ ./stty ispeed 0
relunsec@relunsec:~/software/coreutils/target/debug$ gnustty ispeed 0
gnustty: 'standard input': unable to perform all requested operations
relunsec@relunsec:~/software/coreutils/target/debug$ 
```