fix server response packet decoding for empty password duing connecti…

[dhlin]

…on handshake phase

Reference to the server doc on handshake response 41.
It's synced with mysql current source

Current version seems to be based on this doc.
There is a bit difference on the auth_response part.

How to reproduce:

1. run the program below
2. connect to the server using mysql --user root -p -h 127.0.0.1 --port 4000 databasename

It'll print dbName as atabasename in UseDB, missing the first character

package main 

import (
    "fmt"
    "github.com/siddontang/go-mysql/server"
    "net"
)

type handler struct{
	empty server.EmptyHandler
}

func (h handler) UseDB(dbName string) error {
	fmt.Println(dbName)
	return h.empty.UseDB(dbName)
}

func (h handler) HandleQuery(query string) (*mysql.Result, error) {
	return h.empty.HandleQuery(query)
}

func (h handler) HandleFieldList(table string, fieldWildcard string) ([]*mysql.Field, error) {
	return h.empty.HandleFieldList(table, fieldWildcard)
}

func (h handler) HandleStmtPrepare(query string) (params int, columns int, context interface{}, err error) {
	return h.empty.HandleStmtPrepare(query)
}

func (h handler) HandleStmtExecute(context interface{}, query string, args []interface{}) (*mysql.Result, error) {
	return h.empty.HandleStmtExecute(context, query, args)
}

func (h handler) HandleStmtClose(context interface{}) error {
	return h.empty.HandleStmtClose(context)
}

func (h handler) HandleOtherCommand(cmd byte, data []byte) error {
	return h.empty.HandleOtherCommand(cmd, data)
}

func main() {
	l, _ := net.Listen("tcp", "127.0.0.1:4000")
	c, _ := l.Accept()
	conn, _ := server.NewConn(c, "root", "", handler{})
	for {
		conn.HandleCommand()
	}
}