Fix _PyObject_CallFunctionVa(), use the small stack

Issue #28915. Oops, I disabled the small stack to test both code paths. It's
now fixed.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 8447fdb..7da97ac 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2524,8 +2524,7 @@
                          va_list va, int is_size_t)
 {
     PyObject* small_stack[5];
-    /*const Py_ssize_t small_stack_len = Py_ARRAY_LENGTH(small_stack);*/
-    const Py_ssize_t small_stack_len = 0;
+    const Py_ssize_t small_stack_len = Py_ARRAY_LENGTH(small_stack);
     PyObject **stack;
     Py_ssize_t nargs, i;
     PyObject *result;