gh-121791: Check for `NULL` in `MethodDescriptor2_new` in `_testcapi` (#121792)

Modules/_testcapi/vectorcall.c

@@ -348,6 +348,9 @@ static PyObject *
 MethodDescriptor2_new(PyTypeObject* type, PyObject* args, PyObject *kw)
 {
     MethodDescriptor2Object *op = PyObject_New(MethodDescriptor2Object, type);
+    if (op == NULL) {
+        return NULL;
+    }
     op->base.vectorcall = NULL;
     op->vectorcall = MethodDescriptor_vectorcall;
     return (PyObject *)op;