Values from interpolate-hcl can be negative

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**mapbox-gl-js version**: v1.12.0

**browser**: Node, Chrome

### Steps to Trigger Behavior

Create an expression using `interpolate-hcl` between `red` and `white`.

```
const {expression} = require('mapbox-gl/dist/style-spec')
expression.createExpression(['interpolate-hcl', ['linear'], 2 / 9, 0, 'red', 1, 'white'], {type: 'color'}).value.expression
```

Sometimes the values are negative. Perhaps this is due to floating point errors? I'm trying to create a map legend with 10 stops, so often the `label` value is 2/9, 4/9, etc.

### Link to Demonstration
https://jsbin.com/boxanun/edit?html,output

### Expected Behavior
Returned RGB values should be positive, in the range of `0..1`.

### Actual Behavior
RGB values can be negative

Start with the expression.createExpression entry point in mapbox-gl/dist/style-spec and reproduce the interpolate-hcl case using the provided Node or Chrome example. Trace the resulting RGB values and verify the behavior against the expected 0..1 range; the issue is done when the reported interpolation no longer returns negative RGB values.

Written by the indexing model from the issue text.