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.

py/obj.c

@@ -56,7 +56,7 @@ const char *mp_obj_get_type_str(mp_const_obj_t o_in) {
 void mp_obj_print_helper(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
     // There can be data structures nested too deep, or just recursive
     MP_STACK_CHECK();
-#if !NDEBUG
+#ifndef NDEBUG
     if (o_in == NULL) {
         print(env, "(nil)");
         return;

py/obj.h

@@ -61,7 +61,7 @@ typedef struct _mp_obj_base_t mp_obj_base_t;
 // 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)