Since it will now show an error message, we can rewrite /shorten to…

… `shorten.php` as well.

.htaccess

@@ -1,6 +1,6 @@
 RewriteEngine On
 RewriteBase /
-RewriteRule ^shorten(.+)$ shorten.php?$1 [L,QSA]
+RewriteRule ^shorten(.*)$ shorten.php?$1 [L,QSA]
 RewriteCond %{REQUEST_FILENAME} !-f
 RewriteCond %{REQUEST_FILENAME} !-d
 RewriteRule ^(.*)$ index.php?slug=$1 [L,QSA]