libxml: Fix possible null pointer access (regression 2023-05-26).
Pinpointed by gcc 13 warning:
warning: dereference of NULL ‘clone’ [CWE-476] [-Wanalyzer-null-dereference]
* gnulib-local/lib/libxml/tree.c (xmlDOMWrapCloneNode): Assign clone from
cloneAttr.
