Cancel selection timeout if Agent is closed

We would always wait 10 seconds even if the ICE Agent was closed,
instead return early right when the agent is closed.

Closes pion/webrtc#897

selection.go

@@ -27,7 +27,12 @@ type controllingSelector struct {
 func (s *controllingSelector) Start() {
 	s.startTime = time.Now()
 	go func() {
-		time.Sleep(s.agent.candidateSelectionTimeout)
+		select {
+		case <-s.agent.done:
+			return
+		case <-time.After(s.agent.candidateSelectionTimeout):
+		}
+
 		err := s.agent.run(func(a *Agent) {
 			if s.nominatedPair == nil {
 				p := s.agent.getBestValidCandidatePair()