[Bug #21819] Data objects without members should also be frozen

Merged: https://github.com/ruby/ruby/pull/15792

struct.c

@@ -1809,6 +1809,7 @@ rb_data_initialize_m(int argc, const VALUE *argv, VALUE self)
         if (num_members > 0) {
             rb_exc_raise(rb_keyword_error_new("missing", members));
         }
+        OBJ_FREEZE(self);
         return Qnil;
     }
     if (argc > 1 || !RB_TYPE_P(argv[0], T_HASH)) {

test/ruby/test_data.rb

@@ -262,6 +262,7 @@ class TestData < Test::Unit::TestCase
     assert_equal('#<data >', test.inspect)
     assert_equal([], test.members)
     assert_equal({}, test.to_h)
+    assert_predicate(test, :frozen?)
   end
 
   def test_dup