Only use TravelAgent if supplied; Addresses BUKKIT-3466

If there is no TravelAgent assigned, it can not be used.

src/main/java/org/bukkit/event/player/PlayerPortalEvent.java

@@ -28,7 +28,7 @@ public class PlayerPortalEvent extends PlayerTeleportEvent {
     }
 
     public boolean useTravelAgent() {
-        return useTravelAgent;
+        return useTravelAgent && travelAgent != null;
     }
 
     public TravelAgent getPortalTravelAgent() {