Auth: Explicitly select flatname from customvar_flat

The `*` was just a lazy fix. Should be bullet proof now.
(fingers crossed)

library/Icingadb/Common/Auth.php

@@ -203,7 +203,12 @@ trait Auth
 
                 $columns = $query->getColumns();
                 if (empty($columns)) {
-                    $columns = [$flatvaluePath, '*'];
+                    $columns = [
+                        $customVarRelationName
+                            ? $resolver->qualifyColumn('flatname', $customVarRelationName)
+                            : 'flatname',
+                        $flatvaluePath
+                    ];
                 }
 
                 $flatvalue = null;