(legacy) fix system_gateways_edit.php

(cherry picked from commit 30b63eeb)

src/www/system_gateways_edit.php

@@ -325,7 +325,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         }
         // A result of obfuscating the list of gateways is that over here we need to map things back that should
         // be aligned with the configuration. Not going to fix this now.
-        if (isset($id)) {
+        if (isset($a_gateways[$id]['attribute']) && is_numeric($a_gateways[$id]['attribute']) ) {
             $realid = $a_gateways[$id]['attribute'];
         }