Improve completeSegment for safer DOM manipulation

[leog]

Summary

Enhance completeSegment function to handle cases where segment or placeholder nodes may not exist, preventing potential DOM exceptions.

It's related to this kind of error: vercel/next.js#70048

How did you test this change?

Tried to reproduce the original issue but it's very difficult and unpredictable. Since it comes from using Next.js and the loading.tsx Suspense boundary, it's not that straightforward to reproduce.

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