In the `links_add_base()` callback, use a HTTP-aware URL joining function, rather than a filesystem joining function to avoid PHP warnings on certain systems.

Fixes #29950


git-svn-id: https://develop.svn.wordpress.org/trunk@30766 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/formatting.php

@@ -3694,7 +3694,8 @@ function _links_add_base($m) {
 	return $m[1] . '=' . $m[2] .
 		( preg_match( '#^(\w{1,20}):#', $m[3], $protocol ) && in_array( $protocol[1], wp_allowed_protocols() ) ?
 			$m[3] :
-			path_join( $_links_add_base, $m[3] ) )
+			WP_HTTP::make_absolute_url( $m[3], $_links_add_base )
+		)
 		. $m[2];
 }