Fix deprecation notice for passing a null to preg_replace (#39)

Author: kissit

system/core/Common.php

@@ -727,7 +727,7 @@ function remove_invisible_characters($str, $url_encoded = TRUE)
 
 		do
 		{
-			$str = preg_replace($non_displayables, '', $str, -1, $count);
+			$str = preg_replace($non_displayables, '', (string)$str, -1, $count);
 		}
 		while ($count);