Fix crash in extendSelection
Fix crash when doing word selection or line selection
and moving the cursor to the end of the screen.
Konsole tries to read out of bounds when you double
click to select a whole empty line, and hold down the
mouse button and drag to the right.
Note: still crashes with on some extendSelections
such when y=-1
Doesn't crash anymore, selecting the last column works,
both in normal selection, line selection and word selection.
Reviewers: Konsole, hindenburg
Reviewed By: Konsole, hindenburg
Differential Revision: https://phabricator.kde.org/D17757