Use distinct message for CandidatePair unset

Easier when debugging to tell when setting (or unsetting)

agent.go

@@ -568,16 +568,16 @@ func (a *Agent) updateConnectionState(newState ConnectionState) {
 }
 
 func (a *Agent) setSelectedPair(p *CandidatePair) {
-	a.log.Tracef("Set selected candidate pair: %s", p)
-
 	if p == nil {
 		var nilPair *CandidatePair
 		a.selectedPair.Store(nilPair)
+		a.log.Tracef("Unset selected candidate pair")
 		return
 	}
 
 	p.nominated = true
 	a.selectedPair.Store(p)
+	a.log.Tracef("Set selected candidate pair: %s", p)
 
 	a.updateConnectionState(ConnectionStateConnected)
 

udp_mux_universal.go

@@ -183,7 +183,7 @@ func (m *UniversalUDPMuxDefault) GetXORMappedAddr(serverAddr net.Addr, deadline
 	// or wait for already sent request to complete
 	waitAddrReceived, err := m.sendStun(serverAddr)
 	if err != nil {
-		return nil, errSendSTUNPacket
+		return nil, fmt.Errorf("%w: %s", errSendSTUNPacket, err)
 	}
 
 	// block until response was handled by the connWorker routine and XORMappedAddress was updated