Fix SASL body decode

[wyhaya]

let mut bytes = BytesMut::new();
PgWireBackendMessage::Authentication(Authentication::SASLFinal(Bytes::from("Hello")))
        .encode(&mut bytes);
// Panic !!!
PgWireBackendMessage::decode(&mut bytes);

Error

thread 'main' panicked at /Users/.../bytes-1.6.0/src/bytes_mut.rs:386:9:
split_to out of bounds: 9 <= 5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[sunng87]

Thank you!