net/http/fcgi: fix race between serveRequest and cleanUp

Fixes #11552

Change-Id: I87904e9e3fb4bd1fb4c7075a4e2d0151e5bd37df
Reviewed-on: https://go-review.googlesource.com/11890
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/net/http/fcgi/child.go

@@ -289,6 +289,8 @@ func (c *child) serveRequest(req *request, body io.ReadCloser) {
 }
 
 func (c *child) cleanUp() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	for _, req := range c.requests {
 		if req.pw != nil {
 			// race with call to Close in c.serveRequest doesn't matter because