interfaces: fix a crash report

(cherry picked from commit ba9bbebe)

src/www/interfaces.php

@@ -612,7 +612,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
                 do_input_validation($pconfig, $reqdfields, $reqdfieldsn, $input_errors);
                 break;
             case "none":
-                if (is_array($config['virtualip']['vip'])) {
+                if (isset($config['virtualip']['vip'])) {
                     foreach ($config['virtualip']['vip'] as $vip) {
                         if (is_ipaddrv4($vip['subnet']) && $vip['interface'] == $if) {
                             $input_errors[] = gettext("This interface is referenced by IPv4 VIPs. Please delete those before setting the interface to 'none' configuration.");