[3.14] gh-159294: Fix crash when creating an ExceptionGroup fails with MemoryError (GH-159295) (#160505)

Co-authored-by: Christian Aurich Zanettini Martins <christian.aurichzm@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>

Misc/NEWS.d/next/Core_and_Builtins/2026-10-13-06-23-56.gh-issue-159294.9LTsNA.rst

@@ -0,0 +1,3 @@
+Fix a crash in a debug build when creating an :exc:`ExceptionGroup` or
+:exc:`BaseExceptionGroup` fails with :exc:`MemoryError`. Patch by Christian
+Aurich Zanettini Martins.

Objects/exceptions.c

@@ -998,11 +998,11 @@ BaseExceptionGroup_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
          */
         cls = (PyTypeObject*)PyExc_BaseExceptionGroup;
     }
-    PyBaseExceptionGroupObject *self =
-        PyBaseExceptionGroupObject_CAST(BaseException_new(cls, args, kwds));
-    if (!self) {
+    PyObject *op = BaseException_new(cls, args, kwds);
+    if (!op) {
         goto error;
     }
+    PyBaseExceptionGroupObject *self = PyBaseExceptionGroupObject_CAST(op);
 
     self->msg = Py_NewRef(message);
     self->excs = exceptions;