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>

builtin/receive-pack.c

@@ -521,8 +521,6 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 	proc.in = -1;
 	proc.stdout_to_stderr = 1;
 
-	prepare_push_cert_sha1(&proc);
-
 	if (use_sideband) {
 		memset(&muxer, 0, sizeof(muxer));
 		muxer.proc = copy_to_sideband;
@@ -533,6 +531,8 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 		proc.err = muxer.in;
 	}
 
+	prepare_push_cert_sha1(&proc);
+
 	code = start_command(&proc);
 	if (code) {
 		if (use_sideband)