There is a panic

[zhouqiang-cl]

after many log after "packets.go:36: unexpected EOF", ther is a panic

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x7bf2a6]

goroutine 180 [running]: database/sql.(*Conn).grabConn(0x0, 0x175bac0, 0xc0016aede0, 0x1e, 0x7, 0xc002f1db00, 0x4ef75e) /usr/lib/golang/src/database/sql/sql.go:1745 +0x26 database/sql.(*Conn).PrepareContext(0x0, 0x175bac0, 0xc0016aede0, 0xc0023dc9c0, 0x191, 0x7f282833cb00, 0x0, 0x1e) /usr/lib/golang/src/database/sql/sql.go:1801 +0x46 github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).getAndCacheStmt(0xc00043aff0, 0x175bac0, 0xc0016aede0, 0xc0023dc9c0, 0x191, 0xc0023dc9c0, 0x0, 0xc000470380) /home/tidb/go/src/github.com/pingcap/go-ycsb/db/mysql/db.go:170 +0xfa github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).execQuery(0xc00043aff0, 0x175bac0, 0xc0016aede0, 0xc0023dc9c0, 0x191, 0xc000e09000, 0x1f, 0x1f, 0x4f0dab, 0xc000214570) /home/tidb/go/src/github.com/pingcap/go-ycsb/db/mysql/db.go:279 +0x83 github.com/pingcap/go-ycsb/db/mysql.(*mysqlDB).Insert(0xc00043aff0, 0x175bac0, 0xc0016aede0, 0xc0002aa550, 0xa, 0xc0021d7080, 0x17, 0xc0037fc690, 0x0, 0x0) /home/tidb/go/src/github.com/pingcap/go-ycsb/db/mysql/db.go:342 +0x523 github.com/pingcap/go-ycsb/pkg/client.DbWrapper.Insert(0x1764160, 0xc00043aff0, 0x175bac0, 0xc0016aede0, 0xc0002aa550, 0xa, 0xc0021d7080, 0x17, 0xc0037fc690, 0x0, ...) /home/tidb/go/src/github.com/pingcap/go-ycsb/pkg/client/dbwrapper.go:121 +0xe4 github.com/pingcap/go-ycsb/pkg/workload.(*core).DoInsert(0xc0004ae000, 0x175bac0, 0xc0016aede0, 0x1764d00, 0xc00026a000, 0x0, 0x0) /home/tidb/go/src/github.com/pingcap/go-ycsb/pkg/workload/core.go:269 +0x226 github.com/pingcap/go-ycsb/pkg/client.(*worker).run(0xc0000e0480, 0x175bac0, 0xc0016aede0) /home/tidb/go/src/github.com/pingcap/go-ycsb/pkg/client/client.go:133 +0x370 github.com/pingcap/go-ycsb/pkg/client.(*Client).Run.func2(0xc0002a8880, 0xc000102810, 0x3e8, 0x175ba00, 0xc000446100, 0x34) /home/tidb/go/src/github.com/pingcap/go-ycsb/pkg/client/client.go:212 +0x157 created by github.com/pingcap/go-ycsb/pkg/client.(*Client).Run /home/tidb/go/src/github.com/pingcap/go-ycsb/pkg/client/client.go:206 +0x1af