Bug Fixed: avoiding item waiting_user across take/return

It happend once: a user asked to be added to the waiting
list at the same time other user was returning the item.

So the user ended up waiting for the user he was already
using.

app/Http/Controllers/AlertController.php

@@ -22,6 +22,13 @@ class AlertController extends Controller
     public function store(Request $request)
     {
         $item = User::loggedIn()->items()->find(request('item'));
+        if (!$item->used_by) {
+            session()->flash(
+                FlashMessage::PRIMARY, 
+                __('Oh! This item has just being returned. Take it before anyone else!')
+            );
+            return redirect('home');
+        }
         $item->waiting_user_id = Auth::id();
         $item->timestamps = false;
         $item->save();