Opening QSqlDatabase in parallel causes endless loop at shutdown

How to reproduce:

• Open two QSqlDatabase in separate threads in parallel.

What happens:

• On shutdown when QApplication is freed, there is an endless loop.

I found following cause:

• When two SQL connections are created in parallel, QSqlDatabasePrivate::driverDict() is called twice in parallel. This will call qAddPostRoutine(cleanDriverDict); twice and cleanDriverDict will appear twice in the PostRoutines.
• During shutdown, the second cleanDriverDict calls driverDict() again, which reinitializes qDriverDictInit and adds another qAddPostRoutine. So the cleanDriverDict will be called and added again and again.