Stop executing locator's threads when about to shutdown occurred

Mark that about to shutdown has occurred and don't start
any new thread after that.

Fixes: QTCREATORBUG-25318
Change-Id: I14d47c93dd8e8a82f6f68264a03d5831ec95da75
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/coreplugin.cpp

@@ -443,6 +443,7 @@ QString CorePlugin::msgCrashpadInformation()
 ExtensionSystem::IPlugin::ShutdownFlag CorePlugin::aboutToShutdown()
 {
     Find::aboutToShutdown();
+    m_locator->aboutToShutdown();
     m_mainWindow->aboutToShutdown();
     return SynchronousShutdown;
 }