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
```

parse.y

@@ -13228,27 +13228,40 @@ local_push(struct parser_params *p, int toplevel_scope)
     p->lvtbl = local;
 }
 
+static void
+local_free(struct parser_params *p, struct local_vars *local)
+{
+    if (local->used) {
+        vtable_free(local->used);
+    }
+
+# if WARN_PAST_SCOPE
+    while (local->past) {
+        struct vtable *past = local->past;
+        local->past = past->prev;
+        vtable_free(past);
+    }
+# endif
+
+    vtable_free(local->args);
+    vtable_free(local->vars);
+
+    ruby_sized_xfree(local, sizeof(struct local_vars));
+}
+
 static void
 local_pop(struct parser_params *p)
 {
     struct local_vars *local = p->lvtbl->prev;
     if (p->lvtbl->used) {
         warn_unused_var(p, p->lvtbl);
-        vtable_free(p->lvtbl->used);
     }
-# if WARN_PAST_SCOPE
-    while (p->lvtbl->past) {
-        struct vtable *past = p->lvtbl->past;
-        p->lvtbl->past = past->prev;
-        vtable_free(past);
-    }
-# endif
-    vtable_free(p->lvtbl->args);
-    vtable_free(p->lvtbl->vars);
+
+    local_free(p, p->lvtbl);
+    p->lvtbl = local;
+
     CMDARG_POP();
     COND_POP();
-    ruby_sized_xfree(p->lvtbl, sizeof(*p->lvtbl));
-    p->lvtbl = local;
 }
 
 #ifndef RIPPER
@@ -13856,11 +13869,12 @@ rb_ruby_parser_free(void *ptr)
     if (p->tokenbuf) {
         ruby_sized_xfree(p->tokenbuf, p->toksiz);
     }
+
     for (local = p->lvtbl; local; local = prev) {
-        xfree(local->vars);
         prev = local->prev;
-        xfree(local);
+        local_free(p, local);
     }
+
     {
         token_info *ptinfo;
         while ((ptinfo = p->token_info) != 0) {

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