QMetaObject::invokeMethod: don't leak the slot object

QMetaCallEvent references up, so we need to drop by one. In all other
cases, we need to drop the reference to zero and delete.

[ChangeLog][QtCore][QMetaObject] Fixed a memory leak that happened when
the new-style call to invokeMethod() was used.

Task-number: QTBUG-65462
Change-Id: I39332e0a867442d58082fffd15034b99e31c92a1
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/kernel/qmetaobject.cpp

@@ -1491,6 +1491,12 @@ bool QMetaObject::invokeMethod(QObject *obj,
 
 bool QMetaObject::invokeMethodImpl(QObject *object, QtPrivate::QSlotObjectBase *slot, Qt::ConnectionType type, void *ret)
 {
+    struct Holder {
+        QtPrivate::QSlotObjectBase *obj;
+        ~Holder() { obj->destroyIfLastRef(); }
+    } holder = { slot };
+    Q_UNUSED(holder);
+
     if (! object)
         return false;