Would I do it? No. Is it unsafe? Not really.
An egg will not go bad unless it has bacteria in it. The incubation temperature is a perfect temperature for bacteria growth and the egg is great food for bacteria, but bacteria have to be present to grow.
Whether the egg is fertilized or not has nothing to do with whether bacteria are present. If the egg is fertile and developing, and bacteria show up at that age, that egg will not hatch. It will go bad and likely blow up in the incubator, exactly the same as an unfertilized egg.
I would not eat it, not because it is unsafe, but because of the YUK! factor. It just does not feel right. Those would be great to feed to a dog or back to the chickens though. Just sniff it before you open it or cook it. If it smells, gently toss it.