Fix crash in extendSelection

Summary:
Fix crash when doing word selection or line selection
and moving the cursor to the end of the screen.
After 4e09f089
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

BUG: 402452

Test Plan:
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

(cherry picked from commit b85bbaa8)