Make mrb_top_self return a real instance.

src/variable.c

@@ -419,6 +419,7 @@ obj_iv_p(mrb_value obj)
     case MRB_TT_SCLASS:
     case MRB_TT_HASH:
     case MRB_TT_DATA:
+    case MRB_TT_MAIN:
       return TRUE;
     default:
       return FALSE;