py: Check for NDEBUG using #ifdef rather than #if.
Defining NDEBUG (to any value, even 0) disables debugging. Otherwise,
if it's not defined, debugging is enabled.
diff --git a/py/obj.h b/py/obj.h
index 2d50685..1b1f824 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -61,7 +61,7 @@
// For debugging purposes they are all different. For non-debug mode, we alias
// as many as we can to MP_OBJ_NULL because it's cheaper to load/compare 0.
-#if NDEBUG
+#ifdef NDEBUG
#define MP_OBJ_NULL ((mp_obj_t)0)
#define MP_OBJ_STOP_ITERATION ((mp_obj_t)0)
#define MP_OBJ_SENTINEL ((mp_obj_t)4)