Fixed a issue in reseller limit validation.

Author: Till Brehm

interface/lib/classes/validate_reseller.inc.php

@@ -36,7 +36,7 @@ class validate_reseller {
 	function limit_client($field_name, $field_value, $validator) {
 		global $app;
 
-		if($field_value <= 0) {
+		if($field_value != -1 && $field_value <= 0) {
 			return $app->tform->lng('limit_client_error_positive');
 		} else {
 			return '';