with show changes on we can have negative offsets

for counting words in a range of text which has redline deleted
text at the start

Change-Id: I34149822d43f8ee3fbde58a166dad408fb588fce
(cherry picked from commit 93e7ae6f5719c56c90679e46a6e817dedf5c54c6)
Reviewed-on: https://gerrit.libreoffice.org/11610
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>