Fixed a crash that happened when attempting to select text with left key + shift

If you happened to try and start a keyboard-based, left text selection at the beginning of a line of text, the IDE would crash. It's because `UITextInput::setSelection()` was getting a negative argument for `colEnd`.

For the record, while working on something else in UITextInput, I've come across a lot of 'quirky' behavior in regards to keyboard-based text selection. I dunno -- maybe I'll try and address some stuff in the near future.