Fixes Simulation stopping with an duration=0 interval

src/main/java/de/tu_darmstadt/tk/SmartHomeNetworkSim/core/SimulationManager.java

@@ -88,8 +88,10 @@ public class SimulationManager extends Observable {
 	 */
 	private void simulateTimeStep() {
 		// Just simulate if timer is running
-		if (endTime <= currentTime)
+		if (endTime <= currentTime){
 			stopSimulation();
+			return;
+		}
 		if (currentTime + duration <= endTime) {
 			simulateTimeIntervall(currentTime, duration);
 			currentTime += duration;