est_client: check if a cert is present inside the fwtool helper

This was causing devices without a birt cert being present from doing a
sysupgrade.

Signed-off-by: John Crispin <john@phrozen.org>

feeds/tip/cloud_discovery/files/usr/bin/est_client

@@ -139,6 +139,9 @@ function load_operational_ca() {
 }
 
 function fwtool() {
+	if (!fs.stat('/etc/ucentral/cert.pem'))
+		return 0;
+
 	let pipe = fs.popen(`openssl x509 -in /etc/ucentral/cert.pem -noout -issuer`);
 	let issuer = pipe.read("all");
 	pipe.close();