Don't allow registry to be set to readonly (#184)

VM/src/lapi.cpp

@@ -703,6 +703,7 @@ void lua_setreadonly(lua_State* L, int objindex, bool value)
     const TValue* o = index2adr(L, objindex);
     api_check(L, ttistable(o));
     Table* t = hvalue(o);
+    api_check(L, t != hvalue(registry(L)));
     t->readonly = value;
     return;
 }