QHelpContentModel: Fix that the model sometimes ended up empty

When aborting collecting the content items, sometimes the "abort" state
was not cleaned up correctly, so the next run thought it was aborted
too. If that happened at the end of the multiple start/abort cycles that
happen at initialization, the content model ended up empty.

Task-number: QTCREATORBUG-13233
Change-Id: Ia47a6b033cc3fd1f7a02d5194fb6f7ea68a435c5
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>