should use mrb_type() for the sake of MRB_WORD_BOXING

include/mruby.h

@@ -312,7 +312,7 @@ void mrb_gc_mark(mrb_state*,struct RBasic*);
 } while (0)
 void mrb_field_write_barrier(mrb_state *, struct RBasic*, struct RBasic*);
 #define mrb_field_write_barrier_value(mrb, obj, val) do{\
-  if (MRB_TT_HAS_BASIC_P(val.tt)) mrb_field_write_barrier((mrb), (obj), mrb_basic_ptr(val)); \
+  if (MRB_TT_HAS_BASIC_P(mrb_type(val))) mrb_field_write_barrier((mrb), (obj), mrb_basic_ptr(val)); \
 } while (0)
 void mrb_write_barrier(mrb_state *, struct RBasic*);