Add assertion to RVALUE size

src/gc.c

@@ -1609,6 +1609,9 @@ mrb_init_gc(mrb_state *mrb)
 {
   struct RClass *gc;
 
+  mrb_static_assert(sizeof(RVALUE) <= sizeof(void*) * 6,
+                    "RVALUE size must be within 6 words");
+
   gc = mrb_define_module(mrb, "GC");
 
   mrb_define_class_method(mrb, gc, "start", gc_start, MRB_ARGS_NONE());