Fixed incorrect data in multicellular cell indexes

[hhyyrylainen]

which caused a crash when regrowing and then despawning due to incorrect parent indexes for colony cells

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• PR author has checked that this PR works as intended and doesn't
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  (this is important as to not waste the time of Thrive team
  members reviewing this PR)
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[buckly90]

How can I test this PR? Not sure how to reproduce the bug.

[hhyyrylainen]

I tested this by loading up a previous save where I got this crash within 5-10 minutes, suiciding, and then waiting 30 minutes to see if the game crashed.