ls: zero block size leads to "divide by zero" error

When providing `--block-size=0 -l` we panic with:
```
$ cargo run -q ls --block-size=0 -l

thread 'main' panicked at src/uu/ls/src/ls.rs:2691:34:
attempt to divide by zero
```
GNU `ls`, on the other hand, shows an error message:
```
$ ls --block-size=0 -l
ls: invalid --block-size argument '0'
```