Fix runtime exception

(cherry picked from commit f4ed2eb4c9a8e3a5e4a332baf9cfae04f5c8a3b2)

Sources/VLCGoogleDriveController.m

@@ -110,6 +110,9 @@
 
 - (void)downloadFileToDocumentFolder:(GTLDriveFile *)file
 {
+    if (file == nil)
+        return;
+
     if ([file.mimeType isEqualToString:@"application/vnd.google-apps.folder"]) return;
 
     if (!_listOfGoogleDriveFilesToDownload)