gh-148731: Fix crash in Element.iter() on allocation failure

Initialize parent_stack and parent_stack_used before the fallible
PyMem_New call so that elementiter_dealloc does not dereference
an uninitialized pointer when the allocation fails.

Author: timurmamedov1

Misc/NEWS.d/next/Library/2026-06-24-12-00-00.gh-issue-148731.ElItOm.rst

@@ -0,0 +1,2 @@
+Fix crash in :meth:`xml.etree.ElementTree.Element.iter` when memory
+allocation fails during iterator construction.

Modules/_elementtree.c

@@ -2368,6 +2368,9 @@ create_elementiter(elementtreestate *st, ElementObject *self, PyObject *tag,
     if (!it)
         return NULL;
 
+    it->parent_stack = NULL;
+    it->parent_stack_used = 0;
+
     it->sought_tag = Py_NewRef(tag);
     it->gettext = gettext;
     it->root_element = (ElementObject*)Py_NewRef(self);