[3.15] gh-152951 - Fix double DECREF when newblock fails during deque.extend calls (GH-152961) (#152971)

gh-152951 - Fix double DECREF when `newblock` fails during deque.extend calls (GH-152961)
(cherry picked from commit a90576d)

Co-authored-by: Steve Stagg <stestagg@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-07-03-14-54-33.gh-issue-152951.u8tPCI.rst

@@ -0,0 +1,2 @@
+:class:`collections.deque` prevent rare crash when calling ``extend`` under
+high memory pressure conditions.

Modules/_collectionsmodule.c

@@ -512,7 +512,6 @@ deque_extend_impl(dequeobject *deque, PyObject *iterable)
     iternext = *Py_TYPE(it)->tp_iternext;
     while ((item = iternext(it)) != NULL) {
         if (deque_append_lock_held(deque, item, maxlen) == -1) {
-            Py_DECREF(item);
             Py_DECREF(it);
             return NULL;
         }