Check TravelAgent.findOrCreate(Location) for null; Fixes BUKKIT-3590

If a custom TravelAgent is used and returns null for findOrCreate method
a NullPointerException will occur.
Conflicts:
	src/main/java/net/minecraft/server/PlayerList.java

src/main/java/net/minecraft/server/PlayerList.java

@@ -460,6 +460,9 @@ public abstract class PlayerList {
         }
 
         exit = event.useTravelAgent() ? event.getPortalTravelAgent().findOrCreate(event.getTo()) : event.getTo();
+        if (exit == null) {
+            return;
+        }
         exitWorld = ((CraftWorld) exit.getWorld()).getHandle();
 
         Vector velocity = entityplayer.getBukkitEntity().getVelocity();