Issue 24091: Use after free in Element.extend (1)

# 1055        for (i = 0; i < seqlen; i++) {
# (gdb) n
# 1056            PyObject* element = PySequence_Fast_GET_ITEM(seq, i);
# (gdb) n
# 1057            if (!PyObject_IsInstance(element, (PyObject *)&Element_Type)) {
# (gdb) print *element
# $19 = {_ob_next = 0x4060e6fc, _ob_prev = 0x4056cd8c, ob_refcnt = 1, ob_type = 0x406de3e4}
# (gdb) n
# 1066            if (element_add_subelement(self, element) < 0) {
# (gdb) print *element
# $20 = {_ob_next = 0xdbdbdbdb, _ob_prev = 0xdbdbdbdb, ob_refcnt = -606348325, ob_type = 0xdbdbdbdb}
# 
# Fatal Python error: /home/p/Python-3.4.1/Modules/_elementtree.c:267 object at 0x4056c4cc has negative ref count -606348326
# 
# "element" is removed in __getattribute__ method.