don't drop below zero or worse, flow flow flow

class/campaigns.php

@@ -14,7 +14,7 @@ class Campaigns
 
     public function decreaseAvailCountById( $id, $value = 1 )
     {
-        $sql = 'UPDATE `'. DB_PREFIX . '_gebuchte_werbung` SET `menge` = `menge` - :value WHERE `id`= :id';
+        $sql = 'UPDATE `'. DB_PREFIX . '_gebuchte_werbung` SET `menge` = GREATEST(0, `menge` - :value ) WHERE `id`= :id';
         $sql_params = array(
                            ':id' => $id,
                            ':value' => $value