Create QHelpCollectionHandler's query dynamically

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>

src/assistant/help/qhelpcollectionhandler_p.h

@@ -109,12 +109,12 @@ signals:
 
 private:
     bool isDBOpened();
+    void closeDB();
     bool createTables(QSqlQuery *query);
 
-    bool m_dbOpened;
     QString m_collectionFile;
     QString m_connectionName;
-    mutable QSqlQuery m_query;
+    QSqlQuery *m_query = nullptr;
 };
 
 QT_END_NAMESPACE