segmentation violation failed when send txs using bench.go

[lielongxingkong]

error messages below:

2019-08-08 10:00:17.298 |INFO| Master this round. caller=/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:272
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x920a22]

goroutine 128 [running]:
github.com/DSiSc/repository.(*Repository).GetNonce(0x0, 0xbcede5fc74534fa9, 0x673153c197862a8e, 0xbbf6e7e, 0xc42023b930)
/home/zzl/go/src/github.com/DSiSc/repository/repository.go:276 +0x22
github.com/DSiSc/txpool.(*TxPool).getChainNonce(0xc42039c840, 0xbcede5fc74534fa9, 0x673153c197862a8e, 0xbbf6e7e, 0x0)
/home/zzl/go/src/github.com/DSiSc/txpool/txpool.go:208 +0x43
github.com/DSiSc/txpool.(*TxPool).GetTxs(0xc42039c840, 0x0, 0x0, 0x0)
/home/zzl/go/src/github.com/DSiSc/txpool/txpool.go:115 +0x4b2
github.com/DSiSc/producer.(*Producer).assembleBlock(0xc420168e80, 0xc420374550, 0x0, 0x0, 0xdd1040)
/home/zzl/go/src/github.com/DSiSc/producer/producer.go:71 +0x4b
github.com/DSiSc/producer.(*Producer).MakeBlock(0xc420168e80, 0x12, 0x0, 0x0)
/home/zzl/go/src/github.com/DSiSc/producer/producer.go:46 +0x64
github.com/DSiSc/justitia/node.(*Node).blockFactory(0xc42009ac00, 0x0, 0x0, 0x0, 0x0, 0x687c4b8210333c33, 0xb8a42cdbbef23351, 0x3d63dfb4, 0x0, 0xf00800, ...)
/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:276 +0x230
github.com/DSiSc/justitia/node.(*Node).Round(0xc42009ac00)
/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:334 +0x2c8
github.com/DSiSc/justitia/node.(*Node).NextRound(0xc42009ac00, 0x1)
/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:315 +0x2d4
github.com/DSiSc/justitia/node.(*Node).mainLoop(0xc42009ac00)
/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:384 +0x1ce
created by github.com/DSiSc/justitia/node.(*Node).Start
/home/zzl/go/src/github.com/DSiSc/justitia/node/node.go:446 +0x164

[bchain618]

we have fixed the BUG in follow commit, please review:
DSiSc/txpool@39c6fe1