Fix the link endpoint panic.

The panic was happening as the endpoint was trying to send packets with nil
dispatcher. Do not send packets when dispatcher is nil.

Before: http://sponge2/a609cccc-714a-4a83-b488-e52ea59618f9
After: http://sponge2/8cec0b14-7820-4937-8b28-2ecfaecfa773
PiperOrigin-RevId: 769833561

Author: nybidari

pkg/tcpip/link/pipe/pipe.go

@@ -55,7 +55,10 @@ type Endpoint struct {
 }
 
 func (e *Endpoint) deliverPackets(pkts stack.PacketBufferList) {
-	if !e.linked.IsAttached() {
+	e.linked.mu.RLock()
+	d := e.linked.dispatcher
+	e.linked.mu.RUnlock()
+	if d == nil {
 		return
 	}
 
@@ -66,9 +69,6 @@ func (e *Endpoint) deliverPackets(pkts stack.PacketBufferList) {
 		newPkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
 			Payload: pkt.ToBuffer(),
 		})
-		e.linked.mu.RLock()
-		d := e.linked.dispatcher
-		e.linked.mu.RUnlock()
 		d.DeliverNetworkPacket(pkt.NetworkProtocolNumber, newPkt)
 		newPkt.DecRef()
 	}