gh-151815: Fix segfault in template_iter on allocation failure

Initialize stringsiter and interpolationsiter to NULL immediately
after PyObject_GC_New so that templateiter_clear can safely run
Py_CLEAR if a subsequent PyObject_GetIter call fails.

Author: timurmamedov1

Misc/NEWS.d/next/Core_and_Builtins/2026-06-20-14-00-00.gh-issue-151815.TmplIt.rst

@@ -0,0 +1,2 @@
+Fix segfault in t-string iterator deallocation when :c:func:`PyObject_GetIter`
+fails during construction, caused by ``Py_CLEAR`` on uninitialized pointers.

Objects/templateobject.c

@@ -226,6 +226,8 @@ template_iter(PyObject *op)
     if (iter == NULL) {
         return NULL;
     }
+    iter->stringsiter = NULL;
+    iter->interpolationsiter = NULL;
 
     PyObject *stringsiter = PyObject_GetIter(self->strings);
     if (stringsiter == NULL) {