It's not in the diff context, but files_downcast() is called before this
check. If "refs" is NULL, we would have segfaulted before reaching the
check here. And we should never see NULL refs in backend code (frontend
should have caught it).
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>