doveadm backup: Fixed unnecessary mailbox deletion.

Broken by 79490ec. The idea is to delete the mailbox early to avoid
unnecessary work, but we deleted it too early. If the last mail(s) were
deleted instead of just expunging them we deleted the whole mailbox.

src/doveadm/dsync/dsync-mailbox-import.c

@@ -785,9 +785,10 @@ static bool dsync_mailbox_try_save_cur(struct dsync_mailbox_importer *importer,
 		/* add a record for local mail */
 		i_assert(importer->cur_mail != NULL);
 		if (importer->revert_local_changes) {
-			if (save_change == NULL) {
+			if (save_change == NULL &&
+			    importer->cur_mail->uid >= importer->remote_uid_next) {
 				dsync_mailbox_revert_existing_uid(importer, importer->cur_mail->uid,
-					t_strdup_printf("highest than remote's UIDs (remote UIDNEXT=%u)", importer->remote_uid_next));
+					t_strdup_printf("higher than remote's UIDs (remote UIDNEXT=%u)", importer->remote_uid_next));
 				return TRUE;
 			}
 			mail_expunge(importer->cur_mail);