(legacy) fix crashreport PHP Warning: Illegal string offset 'rule' in...

(legacy) fix crashreport PHP Warning:  Illegal string offset 'rule' in /usr/local/www/firewall_nat.php on line 71

(cherry picked from commit 129def90)

src/www/firewall_nat.php

@@ -68,7 +68,7 @@ function delete_id($id, &$array){
     }
 }
 
-if (!is_array($config['nat']['rule'])) {
+if (!isset($config['nat']['rule']) || !is_array($config['nat']['rule'])) {
     $config['nat']['rule'] = array();
 }