[proton-j] SaslSniffer throws exception if given less than 8 bytes.

The SaslSniffer 'transport wrapper' looks at the initial header to determine if Sasl is in use or not. It throws if the transport is fed less than 8 bytes. This seems incorrect since the full 8 may not yet be available at the time, and the full 8 may not even be needed to make a determination.