LanguageClient: prevent accessing freed data on shutdown

Change-Id: I0d6e7abd653acb2ab7409a9cd5e58a2df3b1f259
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientmanager.cpp

@@ -232,6 +232,11 @@ void LanguageClientManager::shutdown()
     });
 }
 
+bool LanguageClientManager::isShuttingDown()
+{
+    return managerInstance->m_shuttingDown;
+}
+
 LanguageClientManager *LanguageClientManager::instance()
 {
     return managerInstance;