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.c b/py/obj.c
index b35ed57..4d363e8 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -56,7 +56,7 @@
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;