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>
