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