QSqlQuery::exec() took for granted that it can dereference driver(),
which should be true for all sane usage; however, it should not crash
if used misguidedly. Added regression test, based on bug report's
reproducer, which crashes without the fix.
Fixes: QTBUG-100037
Pick-to: 6.3 6.2 5.15 5.12
Change-Id: I94600bc60f89e82a1121b418144006a683921a38
Reviewed-by: Andy Shaw <andy.shaw@qt.io>