net/http/fcgi: fix race in child.serve connection read

Guards the connection read with a mutex, because typeStdin
asynchronously and concurrently writes to the underlying conn.

Fixes #41167

Change-Id: Ia2610f4fde0bd4b108c54164095ea293980b0301
Reviewed-on: https://go-review.googlesource.com/c/go/+/252417
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/http/fcgi/child.go

@@ -155,9 +155,12 @@ func (c *child) serve() {
 	defer c.cleanUp()
 	var rec record
 	for {
+		c.conn.mutex.Lock()
 		if err := rec.read(c.conn.rwc); err != nil {
+			c.conn.mutex.Unlock()
 			return
 		}
+		c.conn.mutex.Unlock()
 		if err := c.handleRecord(&rec); err != nil {
 			return
 		}