Panic upon retaining keys in config-gen

[jdknives]

Describe the bug
When running skywire-cli visor gen-config -r --retain-keys, the following panic happens:

INFO [skywire-cli]: No 'output' set; using default path: /home/pi/go/src/github.com/SkycoinProject/skywire-mainnet/skywire-config.json
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4d0270]

goroutine 1 [running]:
github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands/visor.fillInOldKeys(0x14dc640, 0x4d, 0x1481440, 0x14814d0, 0x0)
	/home/pi/go/src/github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands/visor/gen-config.go:89 +0x178
github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands/visor.glob..func8(0xb5fa90, 0x140e8f0, 0x0, 0x2)
	/home/pi/go/src/github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands/visor/gen-config.go:70 +0x1c8
github.com/spf13/cobra.(*Command).execute(0xb5fa90, 0x140e8e0, 0x2, 0x2, 0xb5fa90, 0x140e8e0)
	/home/pi/go/pkg/mod/github.com/spf13/[email protected]/command.go:830 +0x1f4
github.com/spf13/cobra.(*Command).ExecuteC(0xb5ecd0, 0x1, 0x4296c, 0x14201b0)
	/home/pi/go/pkg/mod/github.com/spf13/[email protected]/command.go:914 +0x230
github.com/spf13/cobra.(*Command).Execute(...)
	/home/pi/go/pkg/mod/github.com/spf13/[email protected]/command.go:864
github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands.Execute()
	/home/pi/go/src/github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/commands/root.go:28 +0x20
main.main()
	/home/pi/go/src/github.com/SkycoinProject/skywire-mainnet/cmd/skywire-cli/skywire-cli.go:11 +0x14

Environment information:

• OS: e.g. Linux Raspian
• develop branch

[jdknives]

Fixed by #267