The four ``vsnprintf(buf, sizeof(buf), msg, ap)`` call sites in
``python/libxml.c`` (pythonWarning, pythonError, pythonFatalError,
libxml_xmlErrorFuncHandler) were updated to allocate ``buf`` /
``str`` on the heap via ``xmlMalloc(1024)`` rather than the stack,
but the ``sizeof(buf)`` argument to ``vsnprintf`` was left in place.
On 64-bit platforms ``sizeof(<pointer>)`` is 8, not 1024, so the
formatted message gets truncated to seven characters before the
trailing NUL.

Pass the allocation size literally (``1024``) to match the
``xmlMalloc`` size and add an explanatory comment at each site.