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.
Merge pull request #38 from sco0ter/of810

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