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
+	}
 }