Fix DOS attack from malicious pongs

A double channel close panic was possible if a peer sent back multiple
pongs for every ping.

If the second pong arrived before the ping goroutine deleted its channel
from the map, the channel would be closed twice and so a panic would
ensue.

This fixes that by having the read goroutine send on the ping
goroutine's channel rather than closing it.

Reported via email by Tibor Kálmán @kalmant

Please update to the new release ASAP!

conn_notjs.go

@@ -189,7 +189,7 @@ 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

@@ -271,7 +271,10 @@ 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
 	}