gh-151815: Fix crash in t-string iterator on partial construction under OOM

Initialize templateiter fields to NULL immediately after PyObject_GC_New so
tp_clear/dealloc on the error path does not clear garbage pointers
(OOM-0024 / gh-151763).

Author: Vamsi-klu

Misc/NEWS.d/next/Core_and_Builtins/2026-07-26-12-00-01.gh-issue-151815.d4e5f6.rst

@@ -0,0 +1 @@
+Fix a crash in t-string iteration when creating the iterator fails partway through due to memory pressure.

Objects/templateobject.c

@@ -226,6 +226,9 @@ template_iter(PyObject *op)
     if (iter == NULL) {
         return NULL;
     }
+    iter->stringsiter = NULL;
+    iter->interpolationsiter = NULL;
+    iter->from_strings = 1;
 
     PyObject *stringsiter = PyObject_GetIter(self->strings);
     if (stringsiter == NULL) {
@@ -242,7 +245,6 @@ template_iter(PyObject *op)
 
     iter->stringsiter = stringsiter;
     iter->interpolationsiter = interpolationsiter;
-    iter->from_strings = 1;
     PyObject_GC_Track(iter);
     return (PyObject *)iter;
 }