[3.15] gh-151773: Fix NULL dereference in PyContextVar_Set (GH-151836) (#152009)

gh-151773: Fix NULL dereference in `PyContextVar_Set` (GH-151836)
(cherry picked from commit d35b171)

Co-authored-by: dev <b.chouksey27@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2026-06-21-16-00-00.gh-issue-151773.mN4kRt.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`contextvars.ContextVar.set` when memory allocation
+fails.

Python/context.c

@@ -362,6 +362,9 @@ PyContextVar_Set(PyObject *ovar, PyObject *val)
     Py_XINCREF(old_val);
     PyContextToken *tok = token_new(ctx, var, old_val);
     Py_XDECREF(old_val);
+    if (tok == NULL) {
+        return NULL;
+    }
 
     if (contextvar_set(var, val)) {
         Py_DECREF(tok);