[php-src] PR #24295: Clear the libxml last error in libxml_use_internal_errors(false)

Pull Request: https://github.com/php/php-src/pull/24295 Author: lacatoire libxml_use_internal_errors(false) destroyed the internal error list but left libxml's own last error in place. libxml_get_last_error() falls back to it once the list is gone, so it kept returning an error while libxml_get_errors() was empty, although the documentation says that disabling clears the existing errors. Reset the last error when the list is destroyed. Calling the function again while the internal errors are already disabled leaves it alone.