Make validate.isURLValid more robust
Also make the control flow more explicit while at it.
Patch by: Sverre Rabbelier
<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
{% for location in sitemaps %}<sitemap><loc>{{ location }}</loc></sitemap>{% endfor %}
</sitemapindex>