fix divide by zero error if there's no error in the optimization.

[allenbenz]

Ran into this while running convert against a sketchy model.

[turboderp]

It's really weird that this can happen if the model otherwise works. It implies it was perfectly quantized which rather makes me think something went wrong. Though, in principle, if you're quantizing and already quantized model, perhaps? Idk. Did the converted model work after this?

[allenbenz]

This was a while back, but I do remember it was a merge/finetune and the converted model worked.
I think you are right in that it was probably caused by some layer of model being effectively quantized by some part of the process of it being made.