Ensure _id2ref finds symbols with the correct type

Prior to this commit it was possible to call `ObjectSpace._id2ref` with
an offset static symbol object_id and get back a new, incorrectly tagged
symbol:

```
> sensible_sym = ObjectSpace._id2ref(:a.object_id)
=> :a
> nonsense_sym = ObjectSpace._id2ref(:a.object_id + 40)
=> :a
> sensible_sym == nonsense_sym
=> false
```

`nonsense_sym` ends up tagged with `RUBY_ID_INSTANCE` instead of
`RB_ID_LOCAL`. That means we can do silly things like:

```
> foo = Object.new
> foo.instance_variable_set(:a, 123)
(irb):2:in `instance_variable_set': `a' is not allowed as an instance variable name (NameError)
> foo.instance_variable_set(ObjectSpace._id2ref(:a.object_id + 40), 123)
=> 123
> foo.instance_variables
=> [:a]
```

This was happening because `get_id_entry` ignores the tag bits when
looking up the symbol. So `rb_id2str(symid)` would return a value and
then we'd continue on with the nonsense `symid`.

This commit prevents the situation by checking that the `symid` actually
matches what we get back from `get_id_entry`. Now we get a `RangeError`
for the nonsense id:

```
> ObjectSpace._id2ref(:a.object_id)
=> :a
> ObjectSpace._id2ref(:a.object_id + 40)
(irb):1:in `_id2ref': 0x000000000013f408 is not symbol id value (RangeError)
```

Co-authored-by: John Hawthorn <jhawthorn@github.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/test/ruby/test_objectspace.rb b/test/ruby/test_objectspace.rb
index e0f9eecd11..3c912fe300 100644
--- a/test/ruby/test_objectspace.rb
+++ b/test/ruby/test_objectspace.rb
@@ -65,6 +65,11 @@ End
     assert_raise_with_message(TypeError, msg) {ObjectSpace._id2ref(Object.new)}
   end
 
+  def test_id2ref_invalid_symbol_id
+    msg = /is not symbol id value/
+    assert_raise_with_message(RangeError, msg) { ObjectSpace._id2ref(:a.object_id + 40) }
+  end
+
   def test_count_objects
     h = {}
     ObjectSpace.count_objects(h)