Add an assertion to deleteLater() before it modifies QEvent::d

The deferred deletion functionality stores the event loop level
nesting count in the QEvent d pointer. In Qt 4, this d pointer was
not usable because we forgot to add a proper copy constructor and
assignment operator to it, so the deleteLater() process stored the
count here safely.

Since Qt 5 now has non-implicit copy methods, the d pointer could be
used in the future. If QEvent uses it, this assertion will
trigger. Note that it doesn't apply to classes derived from QEvent,
though.

Change-Id: I8600c8e9379921e32aca166bc0a6c0b4c4ed799f
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -1074,6 +1074,12 @@ void QCoreApplication::postEvent(QObject *receiver, QEvent *event, int priority)
     if (event->type() == QEvent::DeferredDelete && data == QThreadData::current()) {
         // remember the current running eventloop for DeferredDelete
         // events posted in the receiver's thread
+
+        // check that QEvent's d pointer is unused before we store the loop level
+        // if further updates to QEvent have made the use of the d pointer necessary,
+        // then update this code to store the loop level somewhere else
+        Q_ASSERT_X(event->d == 0, "QCoreApplication::postEvent",
+                   "Internal error: this code relies on QEvent::d being null");
         event->d = reinterpret_cast<QEventPrivate *>(quintptr(data->loopLevel));
     }