[1.0.0] Crash when adding extra legs to spidertron

[micromario]

Steps to reproduce:
1. place spidertron
2. save + exit factorio
3. add another leg to spidertron in prototypes
4. open your save
5. factorio will crash

factorio-current.log

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[Rseding91]

Thanks for the report. This is fixed for some future release. I noticed this issue yesterday while working on some other spider related things and fixed it.