panic when to call stop after pool's task finishes.

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

when we use the pool. if we do like this, it will panic

control.Wait()
control.Stop()

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

succeed

3. What did you see instead (Required)

panic

4. What is your TiDB version? (Required)

master