transport-helper: check dup() return in get_exporter

get_exporter() duplicates helper->in via dup() and stores the
result in fastexport->out. If dup() fails (fd exhaustion), it
returns -1. The child_process machinery interprets out = -1 as
"create a pipe for stdout", which would silently change the
fast-export process's output wiring: instead of sending data
back through the helper's input fd, it would write to a new pipe
that nobody reads from.

Check the return value and report the error before proceeding.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

transport-helper.c

@@ -491,6 +491,8 @@ static int get_exporter(struct transport *transport,
 	/* we need to duplicate helper->in because we want to use it after
 	 * fastexport is done with it. */
 	fastexport->out = dup(helper->in);
+	if (fastexport->out < 0)
+		return error_errno(_("could not dup helper output fd"));
 	strvec_push(&fastexport->args, "fast-export");
 	strvec_push(&fastexport->args, "--use-done-feature");
 	strvec_push(&fastexport->args, data->signed_tags ?