Close ICEGatherer if mux isn't ready

When ICE is completely connected closing the mux is the proper way to
shut everything down. The Close is communicated down each transport.

The ICETransport mux is only available once ICE has completed however.
When shutting down the ICETransport if the mux isn't set close the
ICEGatherer if it has been set.

Resolves #608

peerconnection_close_test.go

@@ -55,3 +55,37 @@ func TestPeerConnection_Close(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// Assert that a PeerConnection that is shutdown before ICE starts doesn't leak
+func TestPeerConnection_Close_PreICE(t *testing.T) {
+	// Limit runtime in case of deadlocks
+	lim := test.TimeOut(time.Second * 20)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	pcOffer, pcAnswer, err := newPair()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	answer, err := pcOffer.CreateOffer(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = pcOffer.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = pcAnswer.SetRemoteDescription(answer); err != nil {
+		t.Fatal(err)
+	}
+
+	err = pcAnswer.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+}