In gitmkdtemp, the return value of mktemp is not tested correctly.
mktemp() always returns its 'template' argument, even upon failure.
An error is signalled by making the template an empty string.
Signed-off-by: Filippo Negroni <fnegroni@flexerasoftware.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>