firewall: fix spurious error when moving alias interface, closes #531

src/www/firewall_virtual_ip_edit.php

@@ -108,10 +108,9 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         if (!is_ipaddr($pconfig['subnet'])) {
             $input_errors[] = gettext("A valid IP address must be specified.");
         } else {
+            $ignore_if = isset($id) ? $a_vip[$id]['interface'] : $pconfig['interface'];
             if ($pconfig['mode'] == 'carp') {
-                $ignore_if = $pconfig['interface']."_vip{$pconfig['vhid']}";
-            } else {
-                $ignore_if = $pconfig['interface'];
+                $ignore_if .= "_vip{$pconfig['vhid']}";
             }
             if (is_ipaddr_configured($pconfig['subnet'], $ignore_if)) {
                 $input_errors[] = gettext("This IP address is being used by another interface or VIP.");