Crash when iterating over lines of empty Rope

[juanibiapina]

https://github.com/an-cabal/an-rope/blob/master/src/lib.rs#L754

This lines blows up with overflow.

[hawkw]

Hi @juanibiapina, thanks for reporting! This line should probably be replaced with saturating arithmetic to avoid the overflow. Eventually, I think there really ought to be a large scale audit of potential overflows.