Memory corruption in sqlite plugin

(I'm just assuming that this issue affects Qt6 as well since the code in the SQLite plugin does not seem to have changed, but I actually didn't verify myself the issue there)

Please see this thread in the mailing list for context and suggestions: https://lists.qt-project.org/pipermail/development/2021-May/041471.html

Valgrind reports a use-after-free in the SQLite Qt plugin, because the string returned by QString::utf16() is passed uncopied to the SQL engine, and then the QString itself gets disposed of.
This sometimes causes the query to not return any results. The program does not crash, though, which makes this issue hard to verify.