Fix backspace-at-index-0 behavior by ignoring negative start indices

[thomsbg]

Reproducible on quilljs.com by placing cursor before first letter and hitting the 'Backspace' key.

Expected behavior: nothing happens.
Observed behavior: the first character is deleted.

[jhchen]

Cool thanks for the catch on the bug. I think I was being too clever in setting start's value in the line above and the code might be more clear if that part was just a normal if/else block. Can you make the change and squash the commits?

[thomsbg]

Do you mean like this?

if hotkey.key == dom.KEYS.BACKSPACE
  start = range.start - 1
else
  start = range.start
@quill.deleteText(start, start + 1, Quill.sources.USER) if start >= 0

Or perhaps this would be better?

start = range.start
start -= 1 if hotkey.key == dom.KEYS.BACKSPACE
@quill.deleteText(start, start + 1, Quill.sources.USER) if start >= 0

[jhchen]

Actually thinking about it more I think the commit is fine. I was thinking something else but I realized there was another issue with it.