gh-159739: Initialize template iterator references before allocating

Backport to 3.15 and 3.14.

Author: JelleZijlstra

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-20-38-17.gh-issue-159739.fc0ed2.rst

@@ -0,0 +1,2 @@
+Avoid a crash if allocation fails while creating a
+:class:`string.templatelib.Template` iterator.

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) {