QPSQL: Fix crash when a prepared statement is deleted after the db was removed

When a prepared statement is still alive after the database was removed
with QSqlDatabase::removeDatabase(), the cleanup routine is trying to
access the driver which is no longer alive which results in a crash.
Fix it by checking if the driver is still alive similar to
5f66486c

.

Fixes: QTBUG-43889
Change-Id: Ib466a76d014e32c055d203bda15b075ad3dff3d9
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Jesus Fernandez <jsfdez@gmail.com>