be more careful with returned table def for schema

lib/schema.php

@@ -107,11 +107,13 @@ class Schema
     {
         $td = $this->getTableDef($table);
 
+        if (!empty($td) && !empty($td->columns)) {
             foreach ($td->columns as $cd) {
                 if ($cd->name == $column) {
                     return $cd;
                 }
             }
+        }
 
         return null;
     }