Chia-Network / chia-blockchain / 23917367233

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[SEC-615] Reject malformed weight proof segments with overflow block at index 0 (#20747)

* Reject malformed weight proof segments with overflow block at index 0

Add a bounds check in __validate_pospace before accessing
segment.sub_slots[idx - 1] on the overflow path. When idx == 0,
Python's negative indexing silently reads the last element instead of a
predecessor slot. This mirrors the existing guard in
_get_challenge_block_vdfs and returns None (same pattern as other
validation failures in this function).

* Add test for overflow block at sub-slot index 0 rejection

Exercises the new idx < 1 guard in __validate_pospace to achieve 100%
diff coverage on the defensive bounds check.

10483 of 11671 branches covered (89.82%)

Branch coverage included in aggregate %.

15 of 15 new or added lines in 2 files covered. (100.0%)

106592 of 116725 relevant lines covered (91.32%)

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