qml: fix MediaCover handler being called after object is destroyed
At least with Qt 6.2 connect
doesn't know when target is destroyed this lead
to "use after free" like scenario.
qrc:/qt/qml/VLC/Widgets/MediaCover.qml:94: TypeError: Value is null and could not be converted to an object
qrc:/qt/qml/VLC/Widgets/MediaCover.qml:86: Error: Invalid write to global property "_loadTimeout"