fix nil pointer when adding a due date (#5587)

* fix nil pointer

* remove nil check and just call loadRepo regardless

Author: adelowo

models/issue_comment.go

@@ -708,6 +708,10 @@ func createDeadlineComment(e *xorm.Session, doer *User, issue *Issue, newDeadlin
 		content = newDeadlineUnix.Format("2006-01-02") + "|" + issue.DeadlineUnix.Format("2006-01-02")
 	}
 
+	if err := issue.LoadRepo(); err != nil {
+		return nil, err
+	}
+
 	return createComment(e, &CreateCommentOptions{
 		Type:    commentType,
 		Doer:    doer,