Cleanup session context for SASL only in _sx_sasl_free.

On authorization failures, the session context does not need to be freed
as the _sx_sasl_free is called anyways. We just need to free it there at
the end of the session's lifetime, otherwise we end up with double free.
Merge pull request #74 from sdebnath/sasl-fix

Cleanup session context for SASL only in _sx_sasl_free.