wgengine/magicsock: validate endpoint.derpAddr in Conn.onUDPRelayAllocResp (#17828)

Otherwise a zero value will panic in Conn.sendUDPStd.

Updates #17827

Signed-off-by: Jordan Whited <jordan@tailscale.com>
(cherry picked from commit 18806de400)

wgengine/magicsock/magicsock.go

@@ -651,7 +651,9 @@ func (c *Conn) onUDPRelayAllocResp(allocResp UDPRelayAllocResp) {
 	ep.mu.Lock()
 	defer ep.mu.Unlock()
 	derpAddr := ep.derpAddr
-	go c.sendDiscoMessage(epAddr{ap: derpAddr}, ep.publicKey, disco.key, allocResp.Message, discoVerboseLog)
+	if derpAddr.IsValid() {
+		go c.sendDiscoMessage(epAddr{ap: derpAddr}, ep.publicKey, disco.key, allocResp.Message, discoVerboseLog)
+	}
 }
 
 // Synchronize waits for all [eventbus] events published