textproc/libxml2: backport upstream commits fixing CVEs

[CVE-2024-56171] Fix use-after-free after xmlSchemaItemListAdd
[CVE-2025-24928] Fix stack-buffer-overflow in xmlSnprintfElements
[CVE-2025-32414] python: Read at most len/4 characters.

PR: 287391
(cherry picked from commit fb3e1d5f3dd216ef419a40570c1a97f1ee28a47f)