Check for null driver() before trying to exec()
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 (cherry picked from commit 78eac57f)