Check AUTHENTICATE command input

Return BAD response on invalid input values (not base64 encoded).
Return BAD response if "*" is received (), see
http://tools.ietf.org/html/rfc3501#section-6.2.2 :
> If the client wishes to cancel an authentication exchange, it issues a line
> consisting of a single "*". If the server receives such a response, it MUST
> reject the AUTHENTICATE command by sending a tagged BAD response.