QHostInfo: fix leaking slot object

We were not ref'ing or deref'ing the slot object in the various places
that owned it. So, if, in the end, the QHostInfoResult object didn't
call the slot we would leak the slot object.

Pick-to: 6.6 6.5 6.2 5.15
Fixes: QTBUG-115263
Change-Id: I45f43756c7589470045d97b59257ccfd85a325b7
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/network/kernel/qhostinfo.cpp

@@ -124,7 +124,6 @@ bool QHostInfoResult::event(QEvent *event)
         // we didn't have a context object, or it's still alive
         if (!withContextObject || receiver)
             slotObj->call(const_cast<QObject*>(receiver.data()), args);
-        slotObj->destroyIfLastRef();
 
         deleteLater();
         return true;

src/network/kernel/qhostinfo_p.h

@@ -51,6 +51,12 @@ public:
             moveToThread(receiver->thread());
     }
 
+    ~QHostInfoResult()
+    {
+        if (slotObj)
+            slotObj->destroyIfLastRef();
+    }
+
     void postResultsReady(const QHostInfo &info);
 
 Q_SIGNALS:
@@ -64,6 +70,8 @@ private:
         : receiver(other->receiver), slotObj(other->slotObj),
           withContextObject(other->withContextObject)
     {
+        if (slotObj)
+            slotObj->ref();
         // cleanup if the application terminates before results are delivered
         connect(QCoreApplication::instance(), &QCoreApplication::aboutToQuit,
                 this, &QObject::deleteLater);