OSV-2021-777 Heap-use-after-free in xmlAddNextSibling

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34461 Crash type: Heap-use-after-free READ 4 Crash state: xmlAddNextSibling xmlXIncludeCopyRange xmlXIncludeCopyXPointer...