stateroot: fix panic on shutdown
Stateroot service is always active, but it might have no wallet.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xc57d41]
goroutine 1 [running]:
github.com/nspcc-dev/neo-go/pkg/wallet.(*Wallet).Close(...)
github.com/nspcc-dev/neo-go/pkg/wallet/wallet.go:175
github.com/nspcc-dev/neo-go/pkg/services/stateroot.(*service).Shutdown(0xc000105880?)
github.com/nspcc-dev/neo-go/pkg/services/stateroot/validators.go:77 +0x81
github.com/nspcc-dev/neo-go/pkg/network.(*Server).Shutdown(0xc000105880)
github.com/nspcc-dev/neo-go/pkg/network/server.go:271 +0x205
github.com/nspcc-dev/neo-go/cli/server.startServer(0xc0002702c0)
github.com/nspcc-dev/neo-go/cli/server/server.go:641 +0x2675
github.com/urfave/cli.HandleAction({0xe456e0?, 0x1155f20?}, 0x4?)
github.com/urfave/cli@v1.22.5/app.go:524 +0x50
github.com/urfave/cli.Command.Run({{0xfca38b, 0x4}, {0x0, 0x0}, {0x0, 0x0, 0x0}, {0xfd6a46, 0x10}, {0xffebe3, ...}, ...}, ...)
github.com/urfave/cli@v1.22.5/command.go:173 +0x65b
github.com/urfave/cli.(*App).Run(0xc000272000, {0xc00003e180, 0x3, 0x3})
github.com/urfave/cli@v1.22.5/app.go:277 +0x8a7
main.main()
./main.go:21 +0x33
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