receive-pack: fix deadlock when we cannot create tmpdir

receive-pack: fix deadlock when we cannot create tmpdir

The err_fd descriptor passed to the unpack() function is
intended to be handed off to the child index-pack, and our
async muxer will read until it gets EOF. However, if we
encounter an error before handing off the descriptor, we
must manually close(err_fd). Otherwise we will be waiting
for our muxer to finish, while the muxer is waiting for EOF
on err_fd.

We fixed an identical deadlock already in 49ecfa13f
(receive-pack: close sideband fd on early pack errors,
2013-04-19). But since then, the function grew a new
early-return in 722ff7f87 (receive-pack: quarantine objects
until pre-receive accepts, 2016-10-03), when we fail to
create a temporary directory. This return needs the same
treatment.

Reported-by: Horst Schirmeier <horst@schirmeier.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>