Write to closed Unix socket on auth failure causes "broken pipe" error

[daniel-nichter]

If connecting to MySQL using a socket file and authentication fails, the driver prints an error like

[MySQL] 2015/12/06 10:18:22 packets.go:118: write unix ->/var/lib/mysql.sock: write: broken pipe

to STDERR because the driver tries to send COM_QUIT on auth failure but MySQL has already closed the connection: https://dev.mysql.com/doc/internals/en/authentication-fails.html

The error is superficial [1], but I've become tired of people asking me "what does it mean?" :-) so I'm attaching a PR that fixes it by not sending COM_QUIT until successfully authenticated, else just close the network connection and do the usual internal cleanup (unset vars, etc.).

[1] Superficial to developers, but to users it raises questions because STDERR keeps us up at night.

Reproduce like:

package main

import (
    "log"
    "database/sql"
    _ "github.com/go-sql-driver/mysql"
)

func main() {
    db, err := sql.Open("mysql", "daniel:wrong@unix(/usr/local/var/mysql/mysql.sock)/")
    if err != nil {
        log.Fatal(err)
    }
    if err := db.Ping(); err != nil {
        log.Fatal(err)
    }
}

Output:

[MySQL] 2015/12/06 12:12:40 packets.go:118: write unix ->/usr/local/var/mysql/mysql.sock: write: broken pipe
2015/12/06 12:12:40 Error 1045: Access denied for user 'daniel'@'localhost' (using password: YES)

Using latest master @ d512f20. Iirc, this issue has always existed in the diver. Not affected by Go version.