SimplePie: Return nothing and throw an error in SimplePie_Sanitize when DOMDocument is disabled.

Note that when SimplePie is used through the WordPress fetch_feed() function, we use kses rather than SimplePie_Sanitize, which removes the dependency on DOMDocument. This change is only for plugins using SimplePie directly.

props rmccue. fixes #21990.



git-svn-id: https://core.svn.wordpress.org/trunk@22970 1a063a9b-81f0-0310-95a4-ce76da25c4cd