forkchoice store: add break in walk back

[tcoratger]

🗒️ Description

@unnawut If the statement if self.blocks[target_block_root].slot > self.blocks[self.safe_target].slot is false then we can just break the loop because it will not be true in the next iteration no?

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[unnawut]

Yep sounds right to me!

I think we can collapse for loop and if/else into a while loop as well but either way works