Fix DOS attack from malicious pongs

Cherry picked from master at 129d303

Author: nhooyr

conn.go

@@ -205,7 +205,7 @@ func (c *Conn) Ping(ctx context.Context) error {
 }
 
 func (c *Conn) ping(ctx context.Context, p string) error {
-	pong := make(chan struct{})
+	pong := make(chan struct{}, 1)
 
 	c.activePingsMu.Lock()
 	c.activePings[p] = pong

read.go

@@ -283,7 +283,10 @@ func (c *Conn) handleControl(ctx context.Context, h header) (err error) {
 		pong, ok := c.activePings[string(b)]
 		c.activePingsMu.Unlock()
 		if ok {
-			close(pong)
+			select {
+			case pong <- struct{}{}:
+			default:
+			}
 		}
 		return nil
 	}