fix possible undefined variable in error-message, thx to Dessa

Signed-off-by: Michael Kaufmann (d00p) <d00p@froxlor.org>

lib/functions/output/function.standard_error.php

@@ -39,6 +39,7 @@ function standard_error($errors = '', $replacer = '')
 		);
 	}
 
+	$link = '';
 	if (isset($_SERVER['HTTP_REFERER']) && strpos($_SERVER['HTTP_REFERER'], $_SERVER['HTTP_HOST']) !== false) {
 		$link = '<a href="'.$_SERVER['HTTP_REFERER'].'">'.$lng['panel']['back'].'</a>';
 	}