QtDeviceUtilites crashes when un-plugging and re-connecting ethernet cable

The commit 5dfaab214d29133bbb2f7e23726f2ef8cd8e8e08 (Do not leak QNetworkSettingsService instances for unnamed networks) introduced a crash when unplugging and reconnecting the ethernet cable in QtDeviceUtilities.

Steps to reproduce (see environment below):

1. Clone qtdeviceutilities repo (https://github.com/qt/qtdeviceutilities.git) and checkout commit 1b549cce0618b1577f9dfac70c1402dbcc3562d9 (Bump version to 5.15.9)
2. Plug in ethernet cable to device
3. Cross compile/deploy and run settingsui
4. Unplug ethernet cable, wait a second and plug-in ethernet cable 
5. Application crashes at:
   1. QNetworkSettingsManagerPrivate::onServicesChanged (line 288)
      1. QNetworkSettingsManagerPrivate::handleNewService(line 308)
         1. QNetworkSettingsServiceModel::append (line 155)
            1. endResetModel()

Root cause:

• When ethernet connection is detected a new QNetworkSettingsService object is created in QNetworkSettingsManagerPrivate::handleNewService. (line 297)
• This new QNetworkSettingsService object is added to the m_serviceModel (line 308)
• When the ethernet cable is un-plugged the service is removed from the model in QNetworkSettingsManagerPrivate::onServicesChanged via the call m_serviceModel->removeService(path) (line 259)
  • QNetworkSettingsServiceModel::removeService calls QNetworkSettingsServiceModel::remove
    • In QNetworkSettingsServiceModel::remove the service is NOT removed from the model and the placeholderFlag is set (line 192)
  • Back in QNetworkSettingsManagerPrivate::onServicesChanged line 265 the service is no scheduled for deletion via service->deleteLater();
  • This leaves a deleted service pointer in the model and causes a crash when the network cable is inserted again. 

Suggested fix:

• Do not delete the service if its a placeholder service in QNetworkSettingsManagerPrivate::onServicesChanged lines 263 and 265.

I attached a suggested patch.