conn: make binds replacable

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/peer.go

@@ -126,13 +126,8 @@ func (peer *Peer) SendBuffer(buffer []byte) error {
 	peer.device.net.RLock()
 	defer peer.device.net.RUnlock()
 
-	if peer.device.net.bind == nil {
-		// Packets can leak through to SendBuffer while the device is closing.
-		// When that happens, drop them silently to avoid spurious errors.
-		if peer.device.isClosed() {
-			return nil
-		}
-		return errors.New("no bind")
+	if peer.device.isClosed() {
+		return nil
 	}
 
 	peer.RLock()