Fix crash when mysql stop (#755)

Co-authored-by: Heran Yang <heran.yang@seafile.com>

common/rpc-service.c

@@ -3873,6 +3873,7 @@ seafile_check_repo_blocks_missing (const char *repo_id,
     free (json_data);
     json_decref (ret_json);
     json_decref (array);
+    seaf_repo_unref (repo);
     return ret;
 }