mod_muc_members_json: Fix potential error when removing old affiliations Found this uncommitted change on a production server... The affiliation data may been `nil` at some point, triggering an error?

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```