dom: fix memory errors when visiting QmlFile::m_astComments

It seems that the m_astComments member of Dom::QmlFile has some memory errors: segmentation faults, asan complains about stack-buffer-underflow but I could not really understand whats going wrong at the moment - probably a dangling reference somewhere.

Reproducer: once you have created a DomItem of a qml file with comments, try to print it out via `qDebug() << item.field(Fields::astComments)` or visit it with a visitor. It will crash when trying to print out the comment.

I used following qml file:

// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
import QtQuick
JSUsages {
    function f() {
        // sanity check: reuse variable names from function f in JSUsages. Those should not appear
        // as usages of the sum of JSUsages.
        let sum = 0;
        sum += 1;
        sum += helloProperty + 32 // valid usage of JSUsages's helloProperty
        return sum
    }}