Don't interpret decayed data as we've failed to send tiny values

Don't interpret decayed data as we've failed to send tiny values

When we're calculating the success probability for min-/max-bucket
pairs and are looking at the 0th' min-bucket, we only look at the
highest max-bucket to decide the success probability. We ignore
max-buckets which have a value below `BUCKET_FIXED_POINT_ONE` to
only consider values which aren't substantially decayed.

However, if all of our data is substantially decayed, this filter
causes us to conclude that the highest max-bucket is bucket zero
even though we really should then be looking at any bucket.

We make this change here, looking at the highest non-zero
max-bucket if no max-buckets have a value above
`BUCKET_FIXED_POINT_ONE`.