Fix crash when extending selection

Summary:
When doing word selection use the previous selection anchor if the new one is out of bounds (user scrolled out of view or similar).

A more proper (but still not really proper) fix than b85bbaa8 reverted by 213afc0e

I have a branch refactoring the word selection stuff a bit, but it's a bit more invasive. This is the minimal change I could get to fix it.

Test Plan: see b85bbaa8 213afc0e 914067d1

Reviewers: hindenburg, pavelkh, ngraham

Reviewed By: hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D18862