Rounding error fix on WeightedIndex::update_weights()

WeightedIndex::update_weights() uses subtraction on old_w. Rounding errors, particularly on f32, may bring total_weight to below 0. This PR makes sure a negative total_weight results in an error.

This replaces PR #955 which was invalidated after a merge. Thanks for your consideration!

src/distributions/weighted_index.rs

@@ -174,7 +174,7 @@ impl<X: SampleUniform + PartialOrd> WeightedIndex<X> {
             total_weight += w;
             prev_i = Some(i);
         }
-        if total_weight == zero {
+        if total_weight <= zero {
             return Err(WeightedError::AllWeightsZero);
         }