Fixed regression in coverart tooltip - The assert occured always

[JoergAtGithub]

This fixes a regression for QT>=5.14 introduced in #4309

If you move the mouse over a cover art in the library, the tool tip should appear, but due to this regression it always asserted. The function parent() returns always nullptr.

Stacktrace:

Qt5Core.dll!00007ffaae1e0598()
Qt5Core.dll!00007ffaae1de80d()
mixxx.exe!mixxx::`anonymous namespace'::handleMessage(QtMsgType type, const QMessageLogContext & context, const QString & input) Line 355
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\util\logging.cpp(355)
[External Code]
[Inline Frame] mixxx.exe!mixxx_debug_assert(const char *) Line 9
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\util\assert.h(9)
[Inline Frame] mixxx.exe!mixxx_debug_assert_return_true(const char *) Line 18
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\util\assert.h(18)
mixxx.exe!BaseTrackTableModel::composeCoverArtToolTipHtml(const QModelIndex & index) Line 521
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\library\basetracktablemodel.cpp(521)
mixxx.exe!BaseTrackTableModel::roleValue(const QModelIndex & index, QVariant && rawValue, int role) Line 573
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\library\basetracktablemodel.cpp(573)
mixxx.exe!BaseTrackTableModel::data(const QModelIndex & index, int role) Line 436
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\library\basetracktablemodel.cpp(436)
[External Code]
mixxx.exe!`anonymous namespace'::runMixxx(MixxxApplication * pApp, const CmdlineArgs & args) Line 72
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\main.cpp(72)
mixxx.exe!main(int argc, char * * argv) Line 200
	at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\main.cpp(200)
[External Code]

[daschuer]

Thank you. Looks and works good.