ctx in extractWorker is not initialized

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

type extractWorker struct {
	ctx                context.Context
	sctx               sessionctx.Context
	isBackgroundWorker bool
	sync.Mutex
}

The later use of ctx will be panic

2. What did you expect to see? (Required)

N/A

3. What did you see instead (Required)

N/A

4. What is your TiDB version? (Required)

master