KTextEditor::ViewPrivate: Avoid warning "Text requested for invalid range"

Summary:
...when cut or copy a line without selection

Reviewers: KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: cullmann, kwrite-devel, kde-frameworks-devel, KTextEditor

Tags: Frameworks, Kate

Differential Revision: https://phabricator.kde.org/D17123