Fixed not being able to wear armour

src/pocketmine/inventory/BaseInventory.php

@@ -461,4 +461,8 @@ abstract class BaseInventory implements Inventory{
 			$player->dataPacket($pk);
 		}
 	}
+
+	public function slotExists(int $slot) : bool{
+		return $slot >= 0 and $slot < $this->slots->getSize(); //use actual slots size to allow PlayerInventory to lie
+	}
 }

src/pocketmine/inventory/Inventory.php

@@ -231,4 +231,12 @@ interface Inventory{
 	 * @param bool $send
 	 */
 	public function onSlotChange(int $index, Item $before, bool $send) : void;
+
+	/**
+	 * Returns whether the specified slot exists in the inventory.
+	 *
+	 * @param int $slot
+	 * @return bool
+	 */
+	public function slotExists(int $slot) : bool;
 }

src/pocketmine/inventory/transaction/action/SlotChangeAction.php

@@ -77,7 +77,7 @@ class SlotChangeAction extends InventoryAction{
 	public function isValid(Player $source) : bool{
 		return (
 			$this->inventorySlot >= 0 and
-			$this->inventorySlot < $this->inventory->getSize() and
+			$this->inventory->slotExists($this->inventorySlot) and
 			$this->inventory->getItem($this->inventorySlot)->equalsExact($this->sourceItem)
 		);
 	}