[php-src] PR #25355: ext/xsl: Fix use-after-free on a nested transform of the same processor

Pull Request: https://github.com/php/php-src/pull/25355 Author: iliaal Starting a second transformation on an XSLTProcessor that is already transforming, from a php:function() callback or from main code while a Fiber is suspended inside one, overwrote the processor's document reference and callback node list under the running transformation, which then read freed memory. Throw an Error for such a call, as importStylesheet() already does. A callback that bails out also no longer leaves the processor marked as transforming, so it stays usable in shutdown functions.