mod_sasl2_fast: Improve handling when SASL profile unexpectedly lacks CB This fixes a traceback reported by riau, but likely does not solve the underlying cause, whatever that is.

2c07bcf56a36d6e74dc0f5422e89bd61f4d31239 0.8-diverge
1656d4fd71d07aa3a52da89d4daf7723a555e7dd last-google-code-commit