fix: proving: Initialize slice with with same length as partition

[rjan90]

Related Issues

Fixes #10564

Proposed Changes

Initialize the postParam.Partitions slice with the same length as i.Partitions before iterating over it in the loop.

Additional Info

Tested on a local net, and now it does not panic:

lotus-miner proving compute window-post 0
Took 1.781950251s
[
  {
    "Deadline": 0,
    "Partitions": [
      {
        "Index": 0,
        "Skipped": []
      }
    ],
    "Proofs": [
      {
        "PoStProof": 5,
        "ProofBytes": "jTEZNQZRWjBRcpnOjyae10RtXnN0k8ALGYeenRLVpzdtBKTQiC+BdVlWdk1qehfarXYYWNE+ib9arhYRj4bmVnyA1xAOlLTzs81lEgW64g1tUOxTVaoBWj0sTPBQGEgbFsMB1vKrVkzBRkTUZU9ZWgaZYb59znxB7/ZrARVJw93hLwd3NRjZQvWNhj/JxfyVgaTuW9Uid2Hc2qIpoDrgKY9HRIQmmKTR9sSjYA3seX6B/6uBDI+/BoqaGJkOcOJc"
      }
    ],
    "ChainCommitEpoch": 0,
    "ChainCommitRand": null
  }
]

Checklist

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