Fix crash in still downloaded ~DocumentData.

Telegram/SourceFiles/data/data_document.cpp

@@ -749,6 +749,7 @@ void DocumentData::automaticLoadSettingsChanged() {
 }
 
 void DocumentData::finishLoad() {
+	// NB! _loader may be in ~FileLoader() already.
 	const auto guard = gsl::finally([&] {
 		destroyLoader();
 	});

Telegram/SourceFiles/storage/file_download.cpp

@@ -43,7 +43,11 @@ FileLoader::FileLoader(
 	Expects(!_filename.isEmpty() || (_size <= Storage::kMaxFileInMemory));
 }
 
-FileLoader::~FileLoader() = default;
+FileLoader::~FileLoader() {
+	if (!_finished) {
+		cancel();
+	}
+}
 
 Main::Session &FileLoader::session() const {
 	return *_session;