Fixes the following crash:

- start gtktiledviewer with a Calc document
- enter edit mode
- create a text selection (selection of multiple cells e.g.)
- leave edit mode -> assert fails

Change-Id: Ic8a65a692679afe1e293f7467f7a72cadcf9f083
Reviewed-on: https://gerrit.libreoffice.org/26266Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>