[Bleeding] Fixed maxStack being incorrectly set to 0 in custom craft inventories. Fixes BUKKIT-1181

By: md_5 <md_5@bigpond.com>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryCustom.java

@@ -27,7 +27,7 @@ public class CraftInventoryCustom extends CraftInventory {
 
     static class MinecraftInventory implements IInventory {
         private ItemStack[] items;
-        private int maxStack;
+        private int maxStack = 64;
         private List<HumanEntity> viewers;
         private String title;
         private InventoryType type;