Initialize m_pageData

Ran in valgrind to confirm that it no longer crashes.
BUG:433532
FIXED-IN:5.21.2

src/page/FacesModel.h

@@ -41,7 +41,7 @@ Q_SIGNALS:
 private:
     void findFaceLoaders(PageDataObject *pageData);
 
-    PageDataObject *m_pageData;
+    PageDataObject *m_pageData = nullptr;
     QVector<FaceLoader *> m_faceLoaders;
 };