Fix memory leak in parser for incomplete tokens

[Bug #19835]

The parser does not free the `tbl` of the `struct vtable` when there are
leftover `lvtbl` in the parser. This causes a memory leak.

The following script reproduces this issue:

```
10.times do
  100_000.times do
    Ripper.parse("class Foo")
  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 2bb307eddf..25ddc381cc 100644
--- a/test/ripper/test_ripper.rb
+++ b/test/ripper/test_ripper.rb
@@ -154,6 +154,13 @@ end
         Ripper.parse("")
       end
     end;
+
+    # [Bug #19835]
+    assert_no_memory_leak(%w(-rripper), "", "#{<<~'end;'}", rss: true)
+      1_000_000.times do
+        Ripper.parse("class Foo")
+      end
+    end;
   end
 
   class TestInput < self