QStandardItemModel: don't leak in mimeData()

When an invalid index is given to QStandardItemModel::mimeData() the
function bails out with a warning and returns a nullptr. But the
allocated data from QAIM::modelData() was not freed.

Pick-to: 6.6 6.5 6.2 5.15
Task-number: QTBUG-116532
Change-Id: Ibce9d51ea09d45f1b0eb3ca980aaff141af5bf68
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/itemmodels/qstandarditemmodel.cpp

@@ -3101,13 +3101,13 @@ QStringList QStandardItemModel::mimeTypes() const
 */
 QMimeData *QStandardItemModel::mimeData(const QModelIndexList &indexes) const
 {
-    QMimeData *data = QAbstractItemModel::mimeData(indexes);
+    std::unique_ptr<QMimeData> data(QAbstractItemModel::mimeData(indexes));
     if (!data)
         return nullptr;
 
     const QString format = qStandardItemModelDataListMimeType();
     if (!mimeTypes().contains(format))
-        return data;
+        return data.release();
     QByteArray encoded;
     QDataStream stream(&encoded, QIODevice::WriteOnly);
 
@@ -3159,7 +3159,7 @@ QMimeData *QStandardItemModel::mimeData(const QModelIndexList &indexes) const
     }
 
     data->setData(format, encoded);
-    return data;
+    return data.release();
 }