Loosen restrictions on inventory sizing

Allows creating (but not opening) inventories created eg via Bukkit.createInventory(..., InventoryType.PLAYER);

src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -1503,13 +1503,13 @@ public final class CraftServer implements Server {
 
     @Override
     public Inventory createInventory(InventoryHolder owner, int size) throws IllegalArgumentException {
-        Validate.isTrue(size % 9 == 0, "Chests must have a size that is a multiple of 9!");
+        Validate.isTrue(9 <= size && size <= 54 && size % 9 == 0, "Size for custom inventory must be a multiple of 9 between 9 and 54 slots");
         return CraftInventoryCreator.INSTANCE.createInventory(owner, size);
     }
 
     @Override
     public Inventory createInventory(InventoryHolder owner, int size, String title) throws IllegalArgumentException {
-        Validate.isTrue(size % 9 == 0, "Chests must have a size that is a multiple of 9!");
+        Validate.isTrue(9 <= size && size <= 54 && size % 9 == 0, "Size for custom inventory must be a multiple of 9 between 9 and 54 slots");
         return CraftInventoryCreator.INSTANCE.createInventory(owner, size, title);
     }
 

src/main/java/org/bukkit/craftbukkit/entity/CraftHumanEntity.java

@@ -333,7 +333,7 @@ public class CraftHumanEntity extends CraftLivingEntity implements HumanEntity {
                         customSize = Containers.GENERIC_9X6;
                         break;
                     default:
-                        throw new IllegalArgumentException("Unsupported custom size " + inventory.getSize());
+                        throw new IllegalArgumentException("Unsupported custom inventory size " + inventory.getSize());
                 }
                 openCustomInventory(inventory, player, customSize);
                 break;

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryCustom.java

@@ -55,7 +55,6 @@ public class CraftInventoryCustom extends CraftInventory {
 
         public MinecraftInventory(InventoryHolder owner, int size, String title) {
             Validate.notNull(title, "Title cannot be null");
-            Validate.isTrue(9 <= size && size <= 54 && size % 9 == 0, "Size for custom inventory must be a multiple of 9 between 9 and 54 slots");
             this.items = NonNullList.a(size, ItemStack.a);
             this.title = title;
             this.viewers = new ArrayList<HumanEntity>();