In case of an empty list, the search for its tail caused a
NULL-pointer dereference.
Signed-off-by: NClemens Buchacher <drizzd@aon.at>
Reported-by: NErik Faye-Lund <kusmabite@googlemail.com>
Acked-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>