Fix `mrb_immediate_p()` definition for `MRB_NO_BOXING`; ref
@@ -177,7 +177,7 @@ struct RCptr {
#endif
#ifndef mrb_immediate_p
-#define mrb_immediate_p(o) (mrb_type(o) < MRB_TT_FREE)
+#define mrb_immediate_p(o) (mrb_type(o) < MRB_TT_CPTR)
#endif
#ifndef mrb_integer_p
#define mrb_integer_p(o) (mrb_type(o) == MRB_TT_INTEGER)
