Fix deleting of QSharedPointer internals in case QPointer loses the race

QPointer uses QWeakPointer / QSharedPointer internals in QObject and has
the code to make sure two threads won't stomp on each other if both try
to create a QPointer for the same QObject at the same time. The
threading code was fine, but had a mistake in the clean up code for the
loser thread: the QtSharedPointer::ExternalRefCountData destructor has a
Q_ASSERT for the state of the reference counts. So we need to set the
state correctly before calling the destructor.

But we don't want to do it in case the Q_ASSERT compiled to nothing. So
we use a hack that violates the Second Rule of Q_ASSERTs: don't do
something with side-effects. This way, we can insert code that will only
be compiled if Q_ASSERTs do something, without having to duplicate the
preprocessor conditions from qglobal.h.

Fixes: QTBUG-71412
Change-Id: I1bd327aeaf73421a8ec5fffd1560fdfc8b73b70c
Reviewed-by: Romain Pokrzywka <romain.pokrzywka@gmail.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>

src/corelib/tools/qsharedpointer.cpp

@@ -1466,6 +1466,9 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge
     x->strongref.store(-1);
     x->weakref.store(2);  // the QWeakPointer that called us plus the QObject itself
     if (!d->sharedRefcount.testAndSetRelease(0, x)) {
+        // ~ExternalRefCountData has a Q_ASSERT, so we use this trick to
+        // only execute this if Q_ASSERTs are enabled
+        Q_ASSERT((x->weakref.store(0), true));
         delete x;
         x = d->sharedRefcount.loadAcquire();
         x->weakref.ref();