LanguageClient: prevent updating closed document

Remove the closed document from the container that tracks document
modifications to prevent accessing a deleted document.

Change-Id: Iafba8ad3ce054761017a8feca28fdc4c8b379ef6
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/languageclient/client.cpp

@@ -647,6 +647,7 @@ void Client::closeDocument(TextEditor::TextDocument *document)
 {
     deactivateDocument(document);
     d->m_postponedDocuments.remove(document);
+    d->m_documentsToUpdate.erase(document);
     if (d->m_openedDocument.remove(document) != 0) {
         handleDocumentClosed(document);
         if (d->m_state == Initialized)