docs: Properly quote self uri in search.php

This removes the classical XSS vulnerability of using unquoted
PHP_SELF.
Reported-by: NJohn Lightsey <john@nixnuts.net>
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/search.php.code.in

@@ -12,7 +12,7 @@
     $querystr = htmlspecialchars($query, ENT_QUOTES, 'UTF-8');
 ?>
 
-<form action="<?php echo $_SERVER['PHP_SELF'], "?query=", rawurlencode($query) ?>"
+<form action="<?php echo htmlspecialchars($_SERVER['PHP_SELF'], ENT_QUOTES, 'UTF-8'), "?query=", rawurlencode($query) ?>"
       enctype="application/x-www-form-urlencoded" method="get">
   <input name="query" type="text" size="50" value="<?php echo $querystr ?>"/>
   <select name="scope">