Validate title for custom inventories. Fixes BUKKIT-4616, BUKKIT-4663

Custom inventories currently do not validate the titles provided. Null values
cause NPEs when writing packets. Values longer than 32 characters
disconnect clients.

This change throws and IllegalArgumentException for null titles or titles
longer than 32 characters.

By: Wesley Wolfe <weswolf@aol.com>

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

@@ -3,6 +3,7 @@ package org.bukkit.craftbukkit.inventory;
 import java.util.ArrayList;
 import java.util.List;
 
+import org.apache.commons.lang.Validate;
 import org.bukkit.craftbukkit.entity.CraftHumanEntity;
 import org.bukkit.entity.HumanEntity;
 import org.bukkit.event.inventory.InventoryType;
@@ -43,6 +44,8 @@ public class CraftInventoryCustom extends CraftInventory {
         }
 
         public MinecraftInventory(InventoryHolder owner, int size, String title) {
+            Validate.notNull(title, "Title cannot be null");
+            Validate.isTrue(title.length() <= 32, "Title cannot be longer than 32 characters");
             this.items = new ItemStack[size];
             this.title = title;
             this.viewers = new ArrayList<HumanEntity>();