Fix QLabel crash if buddy's lifetime is too short

If buddy is deleted then QLabel will still try to deliver the
shortcut event to it, in QLabel::event(), and crash.

For this cherry-pick removed all nice-to-haves and only
fixed dereferencing the null pointer.

Task-Id: QTBUG-66841
Change-Id: Ib6a36a9f99cee31afe1f88eb77f5b6cafb8b1086
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
(cherry picked from commit c082d84d5d7fdbfb64de18f06a8ddbc77f2e6de4)
Reviewed-by: Christoph Schleifenbaum <christoph.schleifenbaum@kdab.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>

src/widgets/widgets/qlabel.cpp

@@ -964,6 +964,8 @@ bool QLabel::event(QEvent *e)
         QShortcutEvent *se = static_cast<QShortcutEvent *>(e);
         if (se->shortcutId() == d->shortcutId) {
             QWidget * w = d->buddy;
+            if (!w)
+                return QFrame::event(e);
             if (w->focusPolicy() != Qt::NoFocus)
                 w->setFocus(Qt::ShortcutFocusReason);
 #if QT_CONFIG(abstractbutton)