Handle potentially-relative base element when getting favicon.

The base element's "href" attribute is not required to be absolute,
so rewrite relative to the site URL if it is relative.

See:
* https://www.w3.org/TR/html51/document-metadata.html#the-base-element
* https://html.spec.whatwg.org/multipage/semantics.html#the-base-element

include/functions.php

@@ -577,7 +577,7 @@
 
 			$base = $xpath->query('/html/head/base');
 			foreach ($base as $b) {
-				$url = $b->getAttribute("href");
+				$url = rewrite_relative_url($url, $b->getAttribute("href"));
 				break;
 			}