adding unique index may cause panic

[wjhuang2016]

Bug Report

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1. Minimal reproduce step (Required)

CREATE TABLE `b1cce552` (
  `f5d9aecb` timestamp DEFAULT '2031-12-22 06:44:52',
  `d9337060` varchar(186) DEFAULT '483272p6glpe87dwj0q8bu2xl83hl688qzff7gssdz2kvwc3mihu4jr4nmdpstp925989w62xu',
  `4c74082f` year(4) DEFAULT '1977',
  `9215adc3` tinytext DEFAULT NULL,
  `85ad5a07` decimal(5,0) NOT NULL DEFAULT '68649',
  `8c60260f` varchar(130) NOT NULL DEFAULT 'drfwe301tuehhkmk0jl79mzekuq0byg',
  `8069da7b` varchar(90) DEFAULT 'ra5rhqzgjal4o47ppr33xqjmumpiiillh7o5ajx7gohmuroan0u',
  `91e218e1` tinytext DEFAULT NULL,
  PRIMARY KEY (`8c60260f`,`85ad5a07`) /*T![clustered_index] CLUSTERED */,
  KEY `d88975e1` (`8069da7b`),
  UNIQUE KEY `09b5cb03` (`85ad5a07`),
  KEY `03fbc867` (`d9337060`),
  KEY `187a655e` (`d9337060`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_bin COMMENT='d8a44b0b';
INSERT INTO `b1cce552` (`f5d9aecb`, `d9337060`, `4c74082f`, `9215adc3`, `85ad5a07`, `8c60260f`, `8069da7b`, `91e218e1`) VALUES ('2031-12-22 06:44:52', '483272p6glpe87dwj0q8bu2xl83hl688qzff7gssdz2kvwc3mihu4jr4nmdpstp925989w62xu', 2028, NULL, 846, 'N6QD1=@ped@owVoJx', '9soPM2d6H', 'Tv%'), ('2031-12-22 06:44:52', '483272p6glpe87dwj0q8bu2xl83hl688qzff7gssdz2kvwc3mihu4jr4nmdpstp925989w62xu', 2028, NULL, 9052, '_HWaf#gD!bw', '9soPM2d6H', 'Tv%');
ALTER TABLE `b1cce552` ADD unique INDEX `65290727` (`4c74082f`, `d9337060`, `8069da7b`);

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

Cancelled normally.

3. What did you see instead (Required)

Panic

4. What is your TiDB version? (Required)

master