Fix 'RFC 6121 8.5.1. No Such User' for IQ stanzas

IQ stanzas, which were routed to a non-existent user were falsely replied to with an IQ-result (e.g. ping).
This fix checks for the existence of the user and returns an error, in case the user does not exist.

Related to OF-810.
I discovered this bug, while checking Openfire for RFC 6121 compliance.

If anybody thinks this deserves its own Jira issue, please comment.