Abort authentication if the client selected an invalid SASL mechanism.

Previously, the server would log an error, but then try to continue with
SCRAM-SHA-256 anyway.

Michael Paquier

Discussion: https://www.postgresql.org/message-id/CAB7nPqR0G5aF2_kc_LH29knVqwvmBc66TF5DicvpGVdke68nKw@mail.gmail.com
