If a link points to a non-existing item in FetchInventory caps, then don't try to add it to the return data rather than suffering an exception later on

OpenSim/Capabilities/Handlers/WebFetchInventoryDescendents/WebFetchInvDescHandler.cs

@@ -301,6 +301,7 @@ namespace OpenSim.Capabilities.Handlers
                                         InventoryItemBase linkedItem
                                             = m_InventoryService.GetItem(new InventoryItemBase(link.AssetID));
 
+                                        if (linkedItem != null)
                                             itemsToReturn.Insert(0, linkedItem);
                                     }
                                 }