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

[cakebaker]

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'

[yrakcaz]

Looking for a first contribution - can I take a stab at this?

[cakebaker]

@yrakcaz sure, go ahead :)