Don't throw error when swapping armor slots into the offhand.

Lots of conflicting info here, but the offhand slot is in fact part of
the quickbar/hotbar (yay bukkit naming) but does *not* use a hotbar "button",
since it has its own hotkey (i.e., F by default instead of 0-8).
The result is a hotbar_swap *action* with a swap_offhand *clicktype*.

Fixes #1698.

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/listener/BlacklistListener.java

@@ -49,6 +49,7 @@
 import org.bukkit.event.EventHandler;
 import org.bukkit.event.block.BlockDispenseArmorEvent;
 import org.bukkit.event.block.BlockDispenseEvent;
+import org.bukkit.event.inventory.ClickType;
 import org.bukkit.event.inventory.InventoryAction;
 import org.bukkit.event.inventory.InventoryClickEvent;
 import org.bukkit.event.inventory.InventoryCreativeEvent;
@@ -58,6 +59,7 @@
 import org.bukkit.event.player.PlayerItemHeldEvent;
 import org.bukkit.inventory.Inventory;
 import org.bukkit.inventory.ItemStack;
+import org.bukkit.inventory.PlayerInventory;
 
 import static com.sk89q.worldguard.bukkit.BukkitUtil.createTarget;
 
@@ -330,6 +332,9 @@ private ItemStack checkEquipped(InventoryClickEvent event) {
                         return cursor;
                     }
                 case HOTBAR_SWAP:
+                    if (event.getClick() == ClickType.SWAP_OFFHAND) {
+                        return clickedInventory == null ? null : ((PlayerInventory) clickedInventory).getItemInOffHand();
+                    }
                     return clickedInventory == null ? null : clickedInventory.getItem(event.getHotbarButton());
                 default:
                     break;