Sanitize Otsu threshold implementation

The current implementation "in certain circumstances can cause an overflow".

See the comment by @landinig [here on the forum](https://forum.image.sc/t/otsus-threshold-method-delivers-different-results-in-scikit-image-and-imagej/24655/18?u=imagejan), and this commit: https://github.com/fiji/Auto_Threshold/commit/cd9684592ef0d7eebddcc118fa4a8a4387fdfe4f.

Start by locating the Otsu threshold implementation and reviewing the linked forum comment and Fiji Auto_Threshold commit. Reproduce the overflow scenario and compare the implementation's results with the referenced behavior. Done means the threshold calculation no longer overflows in those circumstances and existing behavior remains covered by tests.

Written by the indexing model from the issue text.