bugfix: When the fn function is executed, causing panic will cause the key to remain permanently. This leads to a series of problems.

When the fn function is executed, causing panic will cause the key to remain permanently. This leads to a series of problems.

```
goroutine 1

Group.Do
// add
g.m[key] = c

// execute fn()
c.val, c.err = fn() // panic;

// delete key
delete(g.m, key)

goroutine 2

Group.Do
// lookup
if c, ok := g.m[key]; ok {
g.mu.Unlock()
c.wg.Wait() // wait forever
return
```