avoid direct mutation of Item->count field, use Item->pop() instead

I think this change was already applied on the master branch, but I don't remember for sure.

src/pocketmine/item/SpawnEgg.php

@@ -44,7 +44,7 @@ class SpawnEgg extends Item{
 		$entity = Entity::createEntity($this->meta, $player->getLevel(), $nbt);
 
 		if($entity instanceof Entity){
-			--$this->count;
+			$this->pop();
 			$entity->spawnToAll();
 			return true;
 		}