Fix gitmkdtemp: correct test for mktemp() return value

Fix gitmkdtemp: correct test for mktemp() return value

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>