(systemhealth) prevent div by zero, for https://github.com/opnsense/core/issues/1421

(cherry picked from commit 679dd0b8)

src/opnsense/mvc/app/controllers/OPNsense/Diagnostics/Api/SystemhealthController.php

@@ -134,6 +134,7 @@ class SystemhealthController extends ApiControllerBase
             $from_timestamp = $this->getMaxRange($rra_info)["oldest_timestamp"];
             $to_timestamp = $this->getMaxRange($rra_info)["newest_timestamp"];
         }
+        $max_values = ($max_values <=0) ? 1 : $max_values;
 
         $archives = array();
         // find archive match