sasl: fix regression introduced by sasl <-> auth_user integration.

This fixes the null-dereference reported on [full-disclosure][1].  A corrected analysis of the issue
follows below:

When SASL authentication completes, and auth_user is requested, client_p->user may be NULL.  Thusly
accessing fields of client_p->user may cause a null dereference.  In these cases, aborting SASL auth
early is a correct interpretation of the IRCv3.1 specification.  The code must handle this situation,
which this commit corrects.

[1]: http://seclists.org/fulldisclosure/2014/Mar/320