fix `NameError` message

The following code produces two NameErrors respectively
and they are independent, but the second one can show
`private constant` message because of first NameError.

```ruby
class C
  class PrivateClass; end
  private_constant :PrivateClass
end

begin
  eval('class C::PrivateClass; end')
rescue => e
  p e
end

begin
  Object.const_get 'Foo'
rescue => e
  p e
end

  #<NameError: private constant C::PrivateClass referenced>
  #<NameError: private constant C::Foo referenced>
  #=> should be #<NameError: uninitialized constant Foo>
```

It fails the test-all tests with
`make test-all TESTS='ruby/class ruby/parse --seed=58891 -v`.

The reason is clear miss from https://github.com/ruby/ruby/commit/7387c08373a

1 files changed, 11 insertions, 0 deletions

diff --git a/test/ruby/test_class.rb b/test/ruby/test_class.rb
index 5086e60398..d1d196c9af 100644
--- a/test/ruby/test_class.rb
+++ b/test/ruby/test_class.rb
@@ -355,6 +355,17 @@ class TestClass < Test::Unit::TestCase
     assert_equal(42, PrivateClass.new.foo)
   end
 
+  def test_private_const_access
+    assert_raise_with_message NameError, /uninitialized/ do
+      begin
+        eval('class ::TestClass::PrivateClass; end')
+      rescue NameError => e
+      end
+
+      Object.const_get "NOT_AVAILABLE_CONST_NAME_#{__LINE__}"
+    end
+  end
+
   StrClone = String.clone
   Class.new(StrClone)