receive-pack: avoid minor leak in case start_async() fails

If the asynchronous start of copy_to_sideband() fails, then any
env_array entries added to struct child_process proc by
prepare_push_cert_sha1() are leaked.  Call the latter function only
after start_async() succeeded so that the allocated entries are
cleaned up automatically by start_command() or finish_command().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>