Fix memory leak for incomplete lambdas

[Bug #19836]

The parser does not free the chain of `struct vtable`, which causes
memory leaks.

The following script reproduces this issue:

```
10.times do
  100_000.times do
    Ripper.parse("-> {")
  end

  puts `ps -o rss= -p #{$$}`
end
```

1 files changed, 7 insertions, 0 deletions

diff --git a/test/ripper/test_ripper.rb b/test/ripper/test_ripper.rb
index 25ddc381cc..9adfd97dfd 100644
--- a/test/ripper/test_ripper.rb
+++ b/test/ripper/test_ripper.rb
@@ -161,6 +161,13 @@ end
         Ripper.parse("class Foo")
       end
     end;
+
+    # [Bug #19836]
+    assert_no_memory_leak(%w(-rripper), "", "#{<<~'end;'}", rss: true)
+      1_000_000.times do
+        Ripper.parse("-> {")
+      end
+    end;
   end
 
   class TestInput < self