avoid negative probs, closes #2237

[petrelharp]

See discussion at #2237 - maybe there's a better way to do this, but at least this works for this example?

[codecov[bot]]

Codecov Report

Merging #2238 (4b9669f) into main (2d00f51) will not change coverage. The diff coverage is 100.00%.

Additional details and impacted files

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