QMutableEventPoint::detach: deref old private

After detaching, the current QMutableEventPoint obviously doesn't
reference old QMutableEventPointPrivate anymore. Deref it, so that we
do not leak memory.

Change-Id: I3b59667603d41f452eead9a2db13e1d005f622ec
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qevent.cpp

@@ -479,8 +479,10 @@ void QMutableEventPoint::detach()
     if (d->refCount == 1)
         return; // no need: there is only one QEventPoint using it
     qCDebug(lcEPDetach) << "detaching: refCount" << d->refCount << this;
+    auto old = d;
     d = new QEventPointPrivate(*d);
     d->refCount = 1;
+    --old->refCount;
 }
 
 /*! \internal