firewall/nat: fix OpenVPN isset() issues

src/www/firewall_nat_edit.php

@@ -579,8 +579,9 @@ include("head.inc");
 													$interfaces["enc0"] = "IPsec";
 
 											/* add openvpn/tun interfaces */
-											if  ($config['openvpn']["openvpn-server"] || $config['openvpn']["openvpn-client"])
-												$interfaces["openvpn"] = "OpenVPN";
+											if (isset($config['openvpn']['openvpn-server']) || isset($config['openvpn']['openvpn-client'])) {
+												$interfaces['openvpn'] = 'OpenVPN';
+											}
 
 											foreach ($interfaces as $iface => $ifacename): ?>
 											<option value="<?=$iface;?>" <?php if ($iface == $pconfig['interface']) echo "selected=\"selected\""; ?>>