Multi-statements `prefetchPointPlanKeys` panic

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

package main

import (
	"context"
	"database/sql"

	_ "github.com/go-sql-driver/mysql"
)

func main() {
	db, err := sql.Open("mysql", "root@tcp(127.0.0.1:4000)/test")
	mustNil(err)
	defer db.Close()
	ctx := context.Background()
	conn, err := db.Conn(ctx)
	mustNil(err)
	defer conn.Close()
	_, err = conn.ExecContext(ctx, "SET tidb_multi_statement_mode='ON';")
	mustNil(err)
	_, err = conn.ExecContext(ctx, "drop table if exists t;")
	mustNil(err)
	_, err = conn.ExecContext(ctx, `CREATE TABLE t (
		a bigint(20),
		b int(10),
		PRIMARY KEY (b, a),
		UNIQUE KEY uk_a (a)
	  );
	  `)
	mustNil(err)

	_, err = conn.ExecContext(ctx, `insert into t values (1, 1);`)
	mustNil(err)

	_, err = conn.ExecContext(ctx, "BEGIN;")
	mustNil(err)
	_, err = conn.QueryContext(ctx, "delete from t where a = 1; select 1;")
	mustNil(err)
}

func mustNil(err error) {
	if err != nil {
		panic(err)
	}
}

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

1

3. What did you see instead (Required)

Error 1105 (HY000): runtime error: index out of range [7] with length 0

[ERROR] [conn.go:1072] ["connection running loop panic"] [conn=8503385434707984879] [lastSQL="delete from t where a = 1; select 1;"] [err="runtime error: index out of range [7] with length 0"] [stack="github.com/pingcap/tidb/server.(*clientConn).Run.func1\n\t/home/tangenta/tidb/server/conn.go:1075\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:920\nruntime.goPanicIndex\n\t/usr/local/go/src/runtime/panic.go:114\nencoding/binary.bigEndian.Uint64\n\t/usr/local/go/src/encoding/binary/binary.go:179\ngithub.com/pingcap/tidb/tablecodec.DecodeHandleInUniqueIndexValue\n\t/home/tangenta/tidb/tablecodec/tablecodec.go:1650\ngithub.com/pingcap/tidb/server.(*clientConn).prefetchPointPlanKeys\n\t/home/tangenta/tidb/server/conn.go:2063\ngithub.com/pingcap/tidb/server.(*clientConn).handleQuery\n\t/home/tangenta/tidb/server/conn.go:1855\ngithub.com/pingcap/tidb/server.(*clientConn).dispatch\n\t/home/tangenta/tidb/server/conn.go:1372\ngithub.com/pingcap/tidb/server.(*clientConn).Run\n\t/home/tangenta/tidb/server/conn.go:1153\ngithub.com/pingcap/tidb/server.(*Server).onConn\n\t/home/tangenta/tidb/server/server.go:677"]

4. What is your TiDB version? (Required)

v7.1.0, master(33480e8)