Nnn: Segfault opening a new context on an empty dir

[leovilok]

Environment details (Put x in the checkbox along with the information)

[ ] Operating System: Linux
[ ] Desktop Environment: None
[ ] Terminal Emulator:
[ ] Shell:
[ ] Custom desktop opener (if applicable):
[x] Program options used: None
[x] Configuration options set: Nothing
[x] Issue exists on nnn master

Exact steps to reproduce the issue

Open nnn master on an empty dir

~sh
mkdir /tmp/empty
nnn /tmp/empty
~

Then press 2 to open context 2.
-> Segfault.

[0xACE]

It's a bit lazy of a commit, but I did it for you. Cheers.

[leovilok]

Yeah I was lazy tonight, thank you!

[jarun]

I added a guard based on ndents.