git_mkstemps_mode: don't set errno to EINVAL on exit.

git_mkstemps_mode: don't set errno to EINVAL on exit.

When reaching the end of git_mkstemps_mode, at least one call to open()
has been done, and errno has been set accordingly. Setting errno is
therefore not necessary, and actually harmfull since callers can't
distinguish e.g. permanent failure from ENOENT, which can just mean that
we need to create the containing directory.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>