repo: fix potential null pointer dereference in mirror sync (#8065)

internal/db/mirror.go

@@ -324,7 +324,7 @@ func SyncMirrors() {
 
 		m, err := GetMirrorByRepoID(com.StrTo(repoID).MustInt64())
 		if err != nil {
-			log.Error("GetMirrorByRepoID [%d]: %v", m.RepoID, err)
+			log.Error("GetMirrorByRepoID [%v]: %v", repoID, err)
 			continue
 		}