prevent panic on master -> slave transcoder transition

The Stop() method on the liquidsoap controller could have been called
multiple times.

node/node.go

@@ -584,6 +584,7 @@ func (t *transcoder) run() {
 				running = true
 			} else if running {
 				t.liquidsoap.Stop()
+				running = false
 			}
 		case <-t.stop:
 			if running {