Data Race from Update not having protected condition.

[jeffbean]

go test -race -count 1000 -v . -run TestWriteRace

func TestWriteRace(t *testing.T) {
	outBuffer := &bytes.Buffer{}
	totalCount := 20
	bar := New(totalCount)
        bar.Output = outBuffer
	bar.Start()
	var wg sync.WaitGroup
	for i := 0; i < totalCount; i++ {
		wg.Add(1)
		go func() {
			bar.Increment()
			time.Sleep(250 * time.Millisecond)
			wg.Done()
		}()
	}
	wg.Wait()
	bar.Finish()
}

==================
WARNING: DATA RACE
Read at 0x00c420186358 by goroutine 31:
  runtime.slicecopy()
      /usr/local/Cellar/go/1.8/libexec/src/runtime/slice.go:160 +0x0
  bytes.(*Buffer).grow()
      /usr/local/Cellar/go/1.8/libexec/src/bytes/buffer.go:110 +0x492
  bytes.(*Buffer).Write()
      /usr/local/Cellar/go/1.8/libexec/src/bytes/buffer.go:137 +0x60
  fmt.Fprintln()
      /usr/local/Cellar/go/1.8/libexec/src/fmt/print.go:248 +0xad
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).Finish.func1()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:254 +0x141
  sync.(*Once).Do()
      /usr/local/Cellar/go/1.8/libexec/src/sync/once.go:44 +0xe1
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).Finish()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:259 +0x8b
  gopkg.in/cheggaaa/pb%2ev1.TestWriteRace()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb_test.go:42 +0x1b8
  testing.tRunner()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:657 +0x107

Previous write at 0x00c420186358 by goroutine 32:
  runtime.slicecopy()
      /usr/local/Cellar/go/1.8/libexec/src/runtime/slice.go:160 +0x0
  bytes.(*Buffer).Write()
      /usr/local/Cellar/go/1.8/libexec/src/bytes/buffer.go:138 +0xeb
  fmt.Fprint()
      /usr/local/Cellar/go/1.8/libexec/src/fmt/print.go:216 +0xad
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).write()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:414 +0xa60
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).Update()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:445 +0x84
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).refresher()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:472 +0x130

Goroutine 31 (running) created at:
  testing.(*T).Run()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:697 +0x543
  testing.runTests.func1()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:882 +0xaa
  testing.tRunner()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:657 +0x107
  testing.runTests()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:888 +0x4e0
  testing.(*M).Run()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:822 +0x1c3
  main.main()
      gopkg.in/cheggaaa/pb.v1/_test/_testmain.go:68 +0x20f

Goroutine 32 (finished) created at:
  gopkg.in/cheggaaa/pb%2ev1.(*ProgressBar).Start()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb.go:135 +0x15a
  gopkg.in/cheggaaa/pb%2ev1.TestWriteRace()
      /Users/jbean/gocode/src/gopkg.in/cheggaaa/pb.v1/pb_test.go:31 +0xfa
  testing.tRunner()
      /usr/local/Cellar/go/1.8/libexec/src/testing/testing.go:657 +0x107