fix https://github.com/ruby/irb/issues/308
This bug occurred when `dialog.width - calculate_width(s, true)` was negative.
When `dialog.width` is shorter than `old_dialog.width`, it calculates how much padding it has to do. However, there are cases where `s` is longer than `dialog.width`, as in the issue. In that case, `padding_space_with_escape_sequences` will crash.
Here, `old_dialog.width` is longer than `dialog.width`, so I changed the padding width to `old_dialog.width - dialog.width`.
https://github.com/ruby/reline/commit/c581c31e0f
f5829e2935