vm CLI: check whether VM is ready before jumping to the instruction
It allows to avoid panic:
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xdab469]
goroutine 1 [running]:
github.com/nspcc-dev/neo-go/pkg/vm.(*VM).Jump(...)
github.com/nspcc-dev/neo-go/pkg/vm/vm.go:1506
github.com/nspcc-dev/neo-go/pkg/vm/cli.handleRun(0xc0005988f0)
github.com/nspcc-dev/neo-go/pkg/vm/cli/cli.go:413 +0x2e9
github.com/abiosoft/ishell/v2.(*Shell).handleCommand(0xc0004320f0, {0xc00032c7c0, 0xc0002a3920, 0x0})
github.com/abiosoft/ishell/v2@v2.0.2/ishell.go:279 +0x143
github.com/abiosoft/ishell/v2.handleInput(0xc0004320f0, {0xc00032c7c0, 0x3, 0x4})
github.com/abiosoft/ishell/v2@v2.0.2/ishell.go:233 +0x31
github.com/abiosoft/ishell/v2.(*Shell).run(0xc0004320f0)
github.com/abiosoft/ishell/v2@v2.0.2/ishell.go:212 +0x30f
github.com/abiosoft/ishell/v2.(*Shell).Run(0xc0004320f0)
github.com/abiosoft/ishell/v2@v2.0.2/ishell.go:112 +0x28
github.com/nspcc-dev/neo-go/pkg/vm/cli.(*VMCLI).Run(0xc000224030)
github.com/nspcc-dev/neo-go/pkg/vm/cli/cli.go:538 +0x39
github.com/nspcc-dev/neo-go/cli/vm.startVMPrompt(0xc0001f46e0)
github.com/nspcc-dev/neo-go/cli/vm/vm.go:28 +0xb4
github.com/urfave/cli.HandleAction({0xe65fa0, 0x1161c68}, 0x2)
github.com/urfave/cli@v1.22.5/app.go:524 +0xa8
github.com/urfave/cli.Command.Run({{0xfed435, 0x2}, {0x0, 0x0}, {0x0, 0x0, 0x0}, {0x100576d, 0x19}, {0x0, ...}, ...}, ...)
github.com/urfave/cli@v1.22.5/command.go:173 +0x652
github.com/urfave/cli.(*App).Run(0xc0001016c0, {0xc0000c6000, 0x2, 0x2})
github.com/urfave/cli@v1.22.5/app.go:277 +0x705
main.main()
./main.go:19 +0x33
```
913e3878c5