fix: StyledBehindWindowBlur crash in render thread

[RigoLigoRLC]

如果StyledBehindWindowBlur位于Loader中，则在Loader变为非活动状态时，可能触发DQuickBehindWindowBlur渲染任务中引用已释放的对象导致程序崩溃。通过使用QPointer修复了这个问题：在m_item为非法时放弃渲染。

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[deepin-ci-robot]

Hi @RigoLigoRLC. Thanks for your PR. 😃

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743, RigoLigoRLC

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[deepin-bot]

TAG Bot

New tag: 5.7.1
DISTRIBUTION: unstable
Suggest: synchronizing this PR through rebase #401