intel/vulkan: Fix clear_depth_stencil_attachment with unused attachment
When the depth_stencil_attachment is VK_ATTACHMENT_UNUSED, then subpass->depth_stencil_attachment, so check that for NULL before trying to dereference it.
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When the depth_stencil_attachment is VK_ATTACHMENT_UNUSED, then subpass->depth_stencil_attachment, so check that for NULL before trying to dereference it.