Check if entity is in list before decrementing. Fixes BUKKIT-5404

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

@@ -976,10 +976,12 @@ public abstract class World implements IBlockAccess {
 
         // CraftBukkit start - Decrement loop variable field if we've already ticked this entity
         int index = this.entityList.indexOf(entity);
+        if (index != -1) {
             if (index <= this.tickPosition) {
                 this.tickPosition--;
             }
             this.entityList.remove(index);
+        }
         // CraftBukkit end
 
         this.b(entity);