Mark entities as invalid when they get despawned. Addresses BUKKIT-810

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

@@ -143,6 +143,7 @@ public abstract class Entity {
     public boolean cd;
     public boolean ce;
     public UUID uniqueId = UUID.randomUUID(); // CraftBukkit
+    public boolean valid = true; // CraftBukkit
 
     public Entity(World world) {
         this.id = entityCount++;

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

@@ -929,6 +929,8 @@ public class World implements IBlockAccess {
         for (int i = 0; i < this.u.size(); ++i) {
             ((IWorldAccess) this.u.get(i)).b(entity);
         }
+
+        entity.valid = false; // CraftBukkit
     }
 
     public void kill(Entity entity) {

src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java

@@ -212,6 +212,10 @@ public abstract class CraftEntity implements org.bukkit.entity.Entity {
         return !entity.isAlive();
     }
 
+    public boolean isValid() {
+        return entity.isAlive() && entity.valid;
+    }
+
     public Server getServer() {
         return server;
     }