fix incorrect checkups in check_iv_name

src/kernel.c

@@ -564,7 +564,7 @@ check_iv_name(mrb_state *mrb, mrb_sym id)
   int len;
 
   s = mrb_sym2name_len(mrb, id, &len);
-  if (len < 2 || s[0] != '@') {
+  if (len < 2 || !(s[0] == '@' && s[1] != '@')) {
     mrb_name_error(mrb, id, "`%s' is not allowed as an instance variable name", s);
   }
 }