Fix missing argument

This fixes:
[Tue Feb 16 13:50:09 2016] [error] [client 10.48.224.4] PHP Warning:  Missing argument 2 for SimpleSAML_Error_AuthSource::__construct(), called in /apps/simplesamlphp-bd-1.14.0/modules/negotiate/lib/Auth/Source/Negotiate.php on line 336 
[Tue Feb 16 13:50:09 2016] [error] [client 10.48.224.4] PHP Notice:  Undefined variable: reason in /apps/simplesamlphp-bd-1.14.0/lib/SimpleSAML/Error/AuthSource.php(31)