Load issue before accessing index in merge message (#22822) (#22830)

Backport #22822

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

services/pull/merge.go

@@ -99,6 +99,9 @@ func GetDefaultMergeMessage(baseGitRepo *git.Repository, pr *issues_model.PullRe
 				}
 				for _, ref := range refs {
 					if ref.RefAction == references.XRefActionCloses {
+						if err := ref.LoadIssue(); err != nil {
+							return "", err
+						}
 						closeIssueIndexes = append(closeIssueIndexes, fmt.Sprintf("%s %s%d", closeWord, issueReference, ref.Issue.Index))
 					}
 				}