fix scrolling bug
fix scrolling bug

To reproduce:
1. enable allow scrolling past end of the document
2. scroll past end of document
3. navigate with the text cursor in the last line
4. now the view jumps

It worked in all lines, except the last line in the document.

I'm pretty sure this is the wrong fix, but it worked. Still, please review.
CCMAIL: cad91e990f129423ae0dba0202ac36f5d782c502@kde.org

hlamer/kate,jfmcarreira/kate,hlamer/kate,hlamer/kate,hlamer/kate,hlamer/kate,sandsmark/kate,DickJ/kate,DickJ/kate,hlamer/kate,cmacq2/kate,cmacq2/kate,jfmcarreira/kate,sandsmark/kate,cmacq2/kate,hlamer/kate,sandsmark/kate,DickJ/kate,hlamer/kate,DickJ/kate,sandsmark/kate,jfmcarreira/kate,hlamer/kate,hlamer/kate