Anvil result slot is not a real inventory slot

like the crafting grid result slot, this doesn't actually exist, it's
just somewhere you get the result item from.

src/pocketmine/inventory/AnvilInventory.php

@@ -45,7 +45,7 @@ class AnvilInventory extends ContainerInventory{
 	}
 
 	public function getDefaultSize() : int{
-		return 3; //1 input, 1 material, 1 result
+		return 2; //1 input, 1 material
 	}
 
 	/**