Fix crash due to dangling pointer in MouseAnnotation

Summary:
BUG: 388228

Diff applies to Applications/17.12, and should be easy to merge to master. It's kept quite minimal as suggested by Albert.

Albert also suggested to add a dedicated unit test and I'd agree, but am not yet sure how to do it. The original bug involves several classes, including UI: Document, Page, AddAnnotationCommand, PageView, PageViewAnnotator, MouseAnnotation - to name a few. So a test for the exact bug scenario would become a bigger integration test rather than an isolated unit test. The other approach would be to do a real unit test on MouseAnnotation. But again, MouseAnnotation has nasty dependencies (e.g., needs a parent PageView) which I'd have to mock. Any ideas? I'd be interested in a discussion on this topic.

Test Plan:

1. Load a document (e.g. linked PDF from bug report) and enable highlight toolbar (F6).
2. Create highlight annotation.
3. Move the view port so that the annotation position is right beside the highlight tool icon.
4. Move the mouse over the annotation, and then horizontally left until you reach the tool icon; it's important to stay over the highlight annotation as long as in viewport.
5. Press ctrl-z for undo.
6. Click on highlight tool, move right into the document, create new highlight annotation.
7. Okular doesn't crash.

Reviewers: Okular

Subscribers: aacid, ngraham

Tags: Okular

Differential Revision: https://phabricator.kde.org/D9852