panic when setting rate to lower than the number of tokens used in the last interval

[Kimbsen]

Lets say you have a bucket with rateopts: interval=1s rate=100
You read 50bytes.
Then within the same interval change the rate to say 10
the next call to bucket.insert will return a value of -40 which will cause a panic slice bounds of range in Reader.Read

package main

import (
	"bytes"
	"time"

	"github.com/ryanuber/iocap"
)

func main() {

	bucket := iocap.NewGroup(iocap.RateOpts{
		Interval: time.Second,
		Size:     100,
	})
	r := bucket.NewReader(bytes.NewReader(make([]byte, 1024)))
	p := make([]byte, 50)
	r.Read(p)
	bucket.SetRate(iocap.RateOpts{
		Interval: time.Second,
		Size:     10,
	})
	r.Read(p)
}

=> panic: runtime error: slice bounds out of range [:-40]

I have two solutions to this.

• Either restart the interval on bucket.setRate.
• Or in Reader.Read just return when v := b.insert() is less than 0.

I can pull request either. Which would you prefer?