Fix close canal client panic if use Execute after mysql host down

[tomwei7]

Execute some query when mysql host is down or unavailable will set c.conn = nil and it case panic when close canal client.

go-mysql/canal/canal.go

Lines 521 to 526 in f0df38a

	if mysql.ErrorEqual(err, mysql.ErrBadConn) {
	c.conn.Close()
	c.conn = nil
	continue
	}
	return nil, err