zink: add an assert to catch renderpass optimizing bugs

this should only trigger if tc has a bug

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21800>
diff --git a/src/gallium/drivers/zink/zink_clear.c b/src/gallium/drivers/zink/zink_clear.c
index 826ad3d..8d67577 100644
--- a/src/gallium/drivers/zink/zink_clear.c
+++ b/src/gallium/drivers/zink/zink_clear.c
@@ -80,6 +80,8 @@
       if (buffers & PIPE_CLEAR_STENCIL)
          aspect |= VK_IMAGE_ASPECT_STENCIL_BIT;
 
+      assert(zink_is_zsbuf_used(ctx));
+
       attachments[num_attachments].aspectMask = aspect;
       attachments[num_attachments].clearValue.depthStencil.depth = depth;
       attachments[num_attachments].clearValue.depthStencil.stencil = stencil;