QMediaPlayer: clear current playlist on deletion.

Makes sure the current playlist is unbound when QMediaPlayer is
destroyed.

Change-Id: If25efa67bf79af0326f6125d9615165a2c7dd6bb
Reviewed-by: Jim Hodapp <jim.hodapp@canonical.com>
Reviewed-by: Loïc Molinari <loic.molinari@canonical.com>
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>

src/multimedia/playback/qmediaplayer.cpp

@@ -615,6 +615,8 @@ QMediaPlayer::~QMediaPlayer()
 {
     Q_D(QMediaPlayer);
 
+    d->disconnectPlaylist();
+
     if (d->service) {
         if (d->control)
             d->service->releaseControl(d->control);