Fix crash when accessing the content model.

Fix crash when accessing the content model.

1) Make sure invalidateContents() is called whenever the help engine's
   DB readers are destroyed.
2) In QHelpContentProvider::stopCollecting(), remove all root items, as
   they are invalid now: Their child items reference DB readers that
   are going to be destroyed.

Task-number: QTBUG-18829
Change-Id: Id9cc4ea99c43c36ad5c5d1f04157a68bac79d44a
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>