executor: add placeholder count check in prepare stage

[lysu]

What have you changed? (mandatory)

Now, client will meet error when input parameter placeholder count > 65535 in exec stage
this PR add placeholder num check in prepare stage to return error more quick and more evident and make it compatible with mysql.

What is the type of the changes? (mandatory)

• Improvement (non-breaking change which is an improvement to an existing feature)

How has this PR been tested? (mandatory)

• uint tests
• manual tests

Does this PR affect documentation (docs/docs-cn) update? (mandatory)

no

Does this PR affect tidb-ansible update? (mandatory)

no

Does this PR need to be added to the release notes? (mandatory)

no

Refer to a related PR or issue link (optional)

Benchmark result if necessary (optional)

Add a few positive/negative examples (optional)

run

package main

import "database/sql"

import (
	"bytes"
	"fmt"
	_ "github.com/go-sql-driver/mysql"
)

func main() {
	db, err := sql.Open("mysql", "root:root@tcp(127.0.0.1:3306)/test")
	if err != nil {
		panic(err)
	}
	defer db.Close()

	_, err = db.Exec("drop table if exists t")
	if err != nil {
		panic(err)
	}

	_, err = db.Exec("create table t (v int)")
	if err != nil {
		panic(err)
	}

	const pc = 65535 + 2

	var param [pc]interface{}
	param[0] = 0
	var sqlBuilder bytes.Buffer
	sqlBuilder.WriteString("insert into t values (?)")
	for i := 0; i < pc-1; i++ {
		sqlBuilder.WriteString(",(?)")
		param[i+1] = i
	}

	stmt, err := db.Prepare(sqlBuilder.String())
	if err != nil {
		panic(err)
	}

	result, err := stmt.Exec(param[:]...)
	if err != nil {
		panic(err)
	}

	fmt.Println(result)

}

expect:

panic: Error 1390: Prepared statement contains too many placeholders

goroutine 1 [running]:
main.main()
	test/main.go:41 +0x3d2

but got:

panic: sql: expected 1 arguments, got 65537

goroutine 1 [running]:
main.main()
	test/main.go:46 +0x3c2

---

This change is 

[winoros]

Why change here?

[lysu]

before rebase ...go tool vet will say this shadow err...but after rebase complain is disappeared, orz....

reverted..

[lysu]

/run-all-tests

[XuHuaiyu]

Can we add a test case to cover this?

[XuHuaiyu]

move these 2 lines to line 17

[lysu]

PTAL @XuHuaiyu

[XuHuaiyu]

LGTM

[winoros]

lgtm