Fix creative ArrayIndexOutOfBoundsException. Fixes BUKKIT-4305

When a Player drops an ItemStack while in creative mode by placing it outside
of their inventory window, the slot number in the packet is -1. The check
that was added to avoid throwing InventoryCreativeEvent excessively didn't
take this into account, and would cause an ArrayIndexOutOfBoundsException to
be thrown when attempting to get the slot specified by the packet.

This change shorts the invocation of player.defaultContainer.getSlot(
packet107setcreativeslot.b) to only occur if the slot id is within the range
of the Inventory.  This prevents attempting to get the slot from a location
that is actually outside of the Inventory.

src/main/java/net/minecraft/server/PlayerConnection.java

@@ -1459,10 +1459,7 @@ public class PlayerConnection extends Connection {
             boolean flag3 = itemstack == null || itemstack.getData() >= 0 && itemstack.getData() >= 0 && itemstack.count <= 64 && itemstack.count > 0;
 
             // CraftBukkit start - Call click event
-            if (flag1 || flag) { // Insist on valid slot
-                ItemStack existingItem = this.player.defaultContainer.getSlot(packet107setcreativeslot.slot).getItem();
-                // Client assumes that the server forgets the contents of the inventory.  It doesn't.
-                if (!ItemStack.matches(existingItem, packet107setcreativeslot.b)) {
+            if (flag || (flag1 && !ItemStack.matches(this.player.defaultContainer.getSlot(packet107setcreativeslot.slot).getItem(), packet107setcreativeslot.b))) { // Insist on valid slot
 
                 org.bukkit.entity.HumanEntity player = this.player.getBukkitEntity();
                 InventoryView inventory = new CraftInventoryView(player, player.getInventory(), this.player.defaultContainer);
@@ -1499,7 +1496,6 @@ public class PlayerConnection extends Connection {
                     return;
                 }
             }
-            }
             // CraftBukkit end
 
             if (flag1 && flag2 && flag3) {