avoid using the fmt.Sprint to truncate the string

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

package main

import (
	"fmt"
)

func main() {
	len1 := int32(1000000)
	len2 := int32(1000001)
	sql := "test"
	fmt.Printf("%.*q", len1, sql)
	fmt.Printf("\n")
	fmt.Printf("%.*q", len2, sql)
}

2. What did you expect to see? (Required)

normal output

3. What did you see instead (Required)

has an error message

4. What is your TiDB version? (Required)