It fixes a warning about the use of db from the wrong thread.
Before, the default m_query object was created
in QHelpCollectionHandler constructor which caused the
warning to appear. Now we create m_query object
dynamically instead.

This also fixed a leak in QHelpCollectionHandler::openCollectionFile()
where two early returns, after the database had been opened, neither
closed it nor set m_dbOpened.

Change-Id: I24484d25d9d90b1a5d6fe4a5acb61c834e2b9599
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>