Fix panic about current not being a child of parent in clean_children

[lukechu10]

This is a bit of a hack right now. The right way to do it would be to ensure that current only includes nodes that are children of parent as a pre-condition of the function.

This temporarily fixes website navigation in debug mode.

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (05433da) 61.21% compared to head (520552c) 61.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #639 +/- ## ========================================== - Coverage 61.21% 61.19% -0.02% ========================================== Files 55 55 Lines 9021 9022 +1 ========================================== - Hits 5522 5521 -1 - Misses 3499 3501 +2 ``` | [Files](https://app.codecov.io/gh/sycamore-rs/sycamore/pull/639?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sycamore-rs) | Coverage Δ | | |---|---|---| | [packages/sycamore-core/src/render.rs](https://app.codecov.io/gh/sycamore-rs/sycamore/pull/639?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sycamore-rs#diff-cGFja2FnZXMvc3ljYW1vcmUtY29yZS9zcmMvcmVuZGVyLnJz) | `61.90% <54.54%> (-0.65%)` | :arrow_down: |

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