Crash when QLabel's buddy is deleted but not unset as buddy

If a label has a buddy and the buddy is deleted there will be a crash in QLabel::event() when the buddy is dereferenced.

Fix is in progress at https://codereview.qt-project.org/#/c/222041/

Example:

{noformat}

#include <QtWidgets>

int main(int a, char **b)
{
    QApplication app(a, b);

    QWidget w;
    w.resize(500, 500);
    auto lay = new QVBoxLayout(&w);
    auto le = new QLineEdit();
    auto la = new QLabel("&Test");
    la->setBuddy(le);
    lay->addWidget(la);
    lay->addWidget(le);

    auto but = new QPushButton("Delete buddy");
    lay->addWidget(but);
    QObject::connect(but, &QPushButton::clicked, le, [le] {
        delete le;
        qDebug() << "Deleted line edit. Now use the shortcut alt+t.";
    });

    w.show();

    return app.exec();
}

{noformat}