Race condition with dynamic max

[nsagnett]

Hello,

I perform a race condition when I try to setup a dynamic max (like get requests with pagination for example)

To reproduce:

	var (
		bar    = progressbar.Default(0, "Test dynamic max")
		wg     = &sync.WaitGroup{}
		length = 1000
	)
	wg.Add(length)
	for i := 0; i < length; i++ {
		bar.ChangeMax64(int64(i + 1))
		go func(b *progressbar.ProgressBar) {
			defer b.Add(1)
			defer wg.Done()

			time.Sleep(3 * time.Second)
		}(bar)
		time.Sleep(time.Second)
	}
	wg.Wait()

After checking code, it seems missing mutex around progress bar max when ChangeMax64 called

Thanks!

[schollz]

Could you submit a PR?

[mxey]

#183