Prevent _replModeStart line number from becoming negative
I am not sure this is the same as https://bugs.kde.org/show_bug.cgi?id=458015 and https://bugs.kde.org/show_bug.cgi?id=457920, but it was the only path I could find that results in negative startIndex in call to Screen::writeToStream.
Reproduce crash:
- Enable semantic shell integration
- Run command with output longer than scrollback buffer.
- Before command ends, and without selecting text, copy to clipboard.