Fix UB in tst_QObject::disconnectDoesNotLeakFunctor()

If CountedStruct is passed a GetSenderObject object,
it will attempt to call a member on it from within
its own destructor.

That works usually quite well, but in this test case,
which tests for function object leaks when a connection
is torn down because the sender object is destroyed,
the destruction of the CountedStruct happens when all
connections are severed in ~QObject. At that point,
what used to be a GetSenderObject instance no longer
is one and the call into one of its member functions
invokes undefined behavior.

Fix by making QObject::sender() public by a using
declaration instead of a wrapper function.

Found by UBSan:
  tests/auto/corelib/kernel/qobject/tst_qobject.cpp:6007:104: runtime error: member call on address 0x7ffc6e7538b0 which does not point to an object of type 'GetSenderObject'
  0x7ffc6e7538b0: note: object is of type 'QObject'

Change-Id: Ia973140037b3c1b5a670a8a3949d09b956f40349
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -5987,7 +5987,7 @@ class GetSenderObject : public QObject
 {
     Q_OBJECT
 public:
-    QObject *accessSender() { return sender(); }
+    using QObject::sender; // make public
 
 public Q_SLOTS:
     void triggerSignal() { Q_EMIT aSignal(); }
@@ -6003,8 +6003,8 @@ struct CountedStruct
     CountedStruct(GetSenderObject *sender) : sender(sender) { ++countedStructObjectsCount; }
     CountedStruct(const CountedStruct &o) : sender(o.sender) { ++countedStructObjectsCount; }
     CountedStruct &operator=(const CountedStruct &) { return *this; }
-    // accessSender here allows us to check if there's a deadlock
-    ~CountedStruct() { --countedStructObjectsCount; if (sender != Q_NULLPTR) (void)sender->accessSender(); }
+    // calling sender() here allows us to check if there's a deadlock
+    ~CountedStruct() { --countedStructObjectsCount; if (sender) (void)sender->sender(); }
     void operator()() const { }
 
     GetSenderObject *sender;