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
```

parse.y

@@ -13228,23 +13228,27 @@ local_push(struct parser_params *p, int toplevel_scope)
     p->lvtbl = local;
 }
 
+static void
+vtable_chain_free(struct parser_params *p, struct vtable *table)
+{
+    while (!DVARS_TERMINAL_P(table)) {
+        struct vtable *cur_table = table;
+        table = cur_table->prev;
+        vtable_free(cur_table);
+    }
+}
+
 static void
 local_free(struct parser_params *p, struct local_vars *local)
 {
-    if (local->used) {
+    vtable_chain_free(p, local->used);
-        vtable_free(local->used);
-    }
 
 # if WARN_PAST_SCOPE
-    while (local->past) {
+    vtable_chain_free(p, local->past);
-        struct vtable *past = local->past;
-        local->past = past->prev;
-        vtable_free(past);
-    }
 # endif
 
-    vtable_free(local->args);
+    vtable_chain_free(p, local->args);
-    vtable_free(local->vars);
+    vtable_chain_free(p, local->vars);
 
     ruby_sized_xfree(local, sizeof(struct local_vars));
 }

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