Fix php warning (#3521)

Fix Invalid argument supplied for foreach() on all_nats = false

app/tools/nat/all_nats.php

@@ -35,12 +35,14 @@ else {
 
     // check if we have any policy nat !
     $policy_nat_found = false;
+    if($all_nats !== false) {
     	foreach ($all_nats as $n) {
             if ($n->policy=="Yes") {
                 $policy_nat_found = true;
                 break;
             }
         }
+    }
 
     // table
     print "<table class='table sorted table-striped table-top table-td-top' data-cookie-id-table='nat_table'>";