bpo-37233: use _PY_FASTCALL_SMALL_STACK in method_vectorcall (GH-13974)

Objects/classobject.c

@@ -64,11 +64,17 @@ method_vectorcall(PyObject *method, PyObject *const *args,
         Py_ssize_t nkwargs = (kwnames == NULL) ? 0 : PyTuple_GET_SIZE(kwnames);
         PyObject **newargs;
         Py_ssize_t totalargs = nargs + nkwargs;
+        PyObject *newargs_stack[_PY_FASTCALL_SMALL_STACK];
+        if (totalargs <= (Py_ssize_t)Py_ARRAY_LENGTH(newargs_stack) - 1) {
+            newargs = newargs_stack;
+        }
+        else {
             newargs = PyMem_Malloc((totalargs+1) * sizeof(PyObject *));
             if (newargs == NULL) {
                 PyErr_NoMemory();
                 return NULL;
             }
+        }
         /* use borrowed references */
         newargs[0] = self;
         if (totalargs) { /* bpo-37138: if totalargs == 0, then args may be
@@ -77,8 +83,10 @@ method_vectorcall(PyObject *method, PyObject *const *args,
             memcpy(newargs + 1, args, totalargs * sizeof(PyObject *));
         }
         result = _PyObject_Vectorcall(func, newargs, nargs+1, kwnames);
+        if (newargs != newargs_stack) {
             PyMem_Free(newargs);
         }
+    }
     return result;
 }