nil pointer panic during mysql reconnect

[Aylei]

[mysql] 2020/09/17 18:20:35 connection.go:157: invalid connection
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x8506e6]

goroutine 144 [running]:
database/sql.(*Conn).grabConn(0x0, 0x1923200, 0xc0359824b0, 0x0, 0xe000, 0xc0031c9908, 0x50f80e)
        /usr/local/go/src/database/sql/sql.go:1854 +0x26
database/sql.(*Conn).PrepareContext(0x0, 0x1923200, 0xc0359824b0, 0xc03e1b9800, 0x35c1, 0xc0031c9900, 0x418f30, 0x7f9a7c893748)
        /usr/local/go/src/database/sql/sql.go:1910 +0x46
github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).getAndCacheStmt(0xc00028d2f0, 0x1923200, 0xc0359824b0, 0xc03e1b9800, 0x35c1, 0x7f9a7c893748, 0x0, 0x149a7e0)
        /home/alei/go-ycsb/db/mysql/db.go:180 +0xfb
github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).execQuery(0xc00028d2f0, 0x1923200, 0xc0359824b0, 0xc03e1b9800, 0x35c1, 0xc03eae0000, 0x1130, 0x9d08, 0xc0031c9b0d, 0x13)
        /home/alei/go-ycsb/db/mysql/db.go:293 +0x85
github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).BatchInsert(0xc00028d2f0, 0x1923200, 0xc0359824b0, 0x16b73d6, 0x9, 0xc0200e4000, 0xc8, 0x100, 0xc043382000, 0xc8, ...)
        /home/alei/go-ycsb/db/mysql/db.go:410 +0x885
github.com/pingcap/go-ycsb/pkg/client.DbWrapper.BatchInsert(0x1931c60, 0xc00028d2f0, 0x1923200, 0xc0359824b0, 0x16b73d6, 0x9, 0xc0200e4000, 0xc8, 0x100, 0xc043382000, ...)
        /home/alei/go-ycsb/pkg/client/dbwrapper.go:131 +0x328
github.com/pingcap/go-ycsb/pkg/workload/corev2.(*corev2).DoBatchInsert(0xc0002a6c00, 0x1923200, 0xc0359824b0, 0xc8, 0x1932920, 0xc000404000, 0x0, 0x0)
        /home/alei/go-ycsb/pkg/workload/corev2/corev2.go:211 +0x5c3
github.com/pingcap/go-ycsb/pkg/client.(*worker).run(0xc0001e5ec0, 0x1923200, 0xc0359824b0)
        /home/alei/go-ycsb/pkg/client/client.go:130 +0x34f
github.com/pingcap/go-ycsb/pkg/client.(*Client).Run.func2(0xc0005616f0, 0xc0005140f0, 0x1f4, 0x1923140, 0xc000574400, 0x44)
        /home/alei/go-ycsb/pkg/client/client.go:212 +0x16a
created by github.com/pingcap/go-ycsb/pkg/client.(*Client).Run
        /home/alei/go-ycsb/pkg/client/client.go:206 +0x1af