Bug 20982: Sanitize category to prevent XSS on opac-shelves.pl

Bug 20982: Sanitize category to prevent XSS on opac-shelves.pl

== Test plan ==
1. Go to http://localhost:8080/cgi-bin/koha/opac-shelves.pl?category=function(){window.location.href%20=%20%27https://git.koha-community.org/stats/koha-master/authors.html%27}()
2. Note that you are redirected to another website
3. Apply the patch & restart services
4. Repeat the above and you are not redirected

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>