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, 6 insertions, 0 deletions

diff --git a/symbol.c b/symbol.c
index dc7d72cb90..94fa09d19c 100644
--- a/symbol.c
+++ b/symbol.c
@@ -481,6 +481,12 @@ get_id_entry(ID id, const enum id_entry_type t)
     return get_id_serial_entry(rb_id_to_serial(id), id, t);
 }
 
+int
+rb_static_id_valid_p(ID id)
+{
+    return STATIC_ID2SYM(id) == get_id_entry(id, ID_ENTRY_SYM);
+}
+
 static inline ID
 rb_id_serial_to_id(rb_id_serial_t num)
 {