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: Iab3245eb9fd405f28105aa08a976d9a108b9c70a
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>