QHostInfo: fix remaining slotObj leaks

We failed to delete the slot object when lookUpHost() was called in
these (exceptional) circumstances:

- on a thread with no event dispatcher

- after application shut-down, when the QHostInfoLookupManager
  Q_APPLICATION_STATIC was destroyed already

Fix by adding the missing destroyIfLastRef() calls into these code
paths, too.

Amends ad5eb297e1.

This would be so much easier if we had SlotObjUniquePtr...

Pick-to: 6.6 6.5 6.2 5.15
Task-number: QTBUG-115263
Change-Id: Ief8bf125bc196742c0ce59c1fd87ab93242fc0da
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/network/kernel/qhostinfo.cpp

@@ -742,6 +742,8 @@ int QHostInfo::lookupHostImpl(const QString &name,
 
     if (!QAbstractEventDispatcher::instance(QThread::currentThread())) {
         qWarning("QHostInfo::lookupHost() called with no event dispatcher");
+        if (slotObj)
+            slotObj->destroyIfLastRef();
         return -1;
     }
 
@@ -802,6 +804,8 @@ int QHostInfo::lookupHostImpl(const QString &name,
             QObject::connect(&runnable->resultEmitter, SIGNAL(resultsReady(QHostInfo)),
                                 receiver, member, Qt::QueuedConnection);
         manager->scheduleLookup(runnable);
+    } else if (slotObj) {
+        slotObj->destroyIfLastRef();
     }
 #endif // Q_OS_WASM
     return id;