gst_sparse_file_read() is supposed to set @error when returning 0 but in some cases was not. Hopefully fix a crash in gst_download_buffer_read_buffer() which is checking error->code when 0 is returned. I'm not totally sure when this happens as I debugged this from a post mortem crash but returning a generic error here seems the safe thing to do. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8321>