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>