Fix entities not having valid "isValid" states. Addresses BUKKIT-2366

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

@@ -91,7 +91,7 @@ public abstract class Entity {
     public boolean al;
     public EnumEntitySize am;
     public UUID uniqueId = UUID.randomUUID(); // CraftBukkit
-    public boolean valid = true; // CraftBukkit
+    public boolean valid = false; // CraftBukkit
 
     public Entity(World world) {
         this.id = entityCount++;

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

@@ -878,6 +878,8 @@ public abstract class World implements IBlockAccess {
 
             iworldaccess.a(entity);
         }
+
+        entity.valid = true; // CraftBukkit
     }
 
     protected void b(Entity entity) {