modules/m_sasl: 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