[ticket/11233] prohibit selecting anonymous user as a PM recipient

While composing pm, it should not be allowed to add
anonymous user as a PM recipient

PHPBB3-11233
Merge remote-tracking branch 'dhruvgoel92/ticket/11233' into develop-olympus

* dhruvgoel92/ticket/11233:
  [ticket/11233] prohibit selecting anonymous user as a PM recipient