encoding/xml: stack exhaustion in Unmarshal

Calling Unmarshal on a XML document into a Go struct which has a nested field that uses the `any` field tag can cause a panic due to stack exhaustion.

This is CVE-2022-30633.

(This was a PRIVATE issue tracked in http://b/227341754 and fixed by http://tg/1421319.)

/cc @golang/security and @golang/release 


