lib-index: Don't update log_file_tail_offset on sync if reading dovec…

…ot.index.log failed

This is more of a theoretical issue, since dovecot.index.log read failure is
very unlikely.

src/lib-index/mail-index-sync-update.c

@@ -1051,7 +1051,8 @@ int mail_index_sync_map(struct mail_index_map **_map,
 	   over following external transactions to avoid extra unneeded log
 	   reading. */
 	i_assert(map->hdr.log_file_seq == index->log->head->hdr.file_seq);
-	if (map->hdr.log_file_tail_offset < index->log->head->max_tail_offset) {
+	if (ret == 0 &&
+	    map->hdr.log_file_tail_offset < index->log->head->max_tail_offset) {
 		map->hdr.log_file_tail_offset =
 			index->log->head->max_tail_offset;
 	}