MUC: Fix logic for access to affiliation lists
Fixes https://prosody.im/security/advisory_20210722/
Backs out 4d7b925652d9
For full information on our dependencies, version requirements, and
where to find them, see https://prosody.im/doc/depends
If you have luarocks available on your platform, install the following:
- luaexpat
- luasocket
- luafilesystem
- luasec