Check if user can update the task

Found a bug in Glpi where a user with task create rights, but
without task update rights (that, of course, cannot be assigned to
a task), when create a new task, can get assigned to that task (when
shouldn't). So we must test for task update rights before allow using
the timer's buttons.

inc/task.class.php

@@ -26,7 +26,7 @@ static public function postForm($params) {
 
                $task_id = $item->getID();
                $rand = mt_rand();
-               $buttons = self::checkTech($task_id);
+               $buttons = (self::checkTech($task_id) && $item->can($task_id, UPDATE));
                $time = self::totalEndTime($task_id);
                $text_restart = __('Restart', 'actualtime');
                $text_pause = __('Pause', 'actualtime');