fix bug when a user delete but assigned on issue (#3318)

models/issue_comment.go

@@ -247,15 +247,21 @@ func (c *Comment) LoadAssignees() error {
 	if c.OldAssigneeID > 0 {
 		c.OldAssignee, err = getUserByID(x, c.OldAssigneeID)
 		if err != nil {
+			if !IsErrUserNotExist(err) {
 				return err
 			}
+			c.OldAssignee = NewGhostUser()
+		}
 	}
 
 	if c.AssigneeID > 0 {
 		c.Assignee, err = getUserByID(x, c.AssigneeID)
 		if err != nil {
+			if !IsErrUserNotExist(err) {
 				return err
 			}
+			c.Assignee = NewGhostUser()
+		}
 	}
 	return nil
 }