Fix panic

[ersonp]

This commit fixes the panic by checking if the stunClient is initilised or not before checking the NATType.

Did you run make format && make check?
yes

Fixes #1027

Changes:

• Added check to see if stunClient is not nil

How to test this PR:

1. On in config with test deployment set the vpn-client as

   {
   	"name": "vpn-client",
   	"args": [
   		"-srv",
   		"03e6a435b16de6ac49d78dea29e5f136c1b223a2b7128d61c36f29eef343040ad2"
   	],
   	"auto_start": true,
   	"port": 43
   },

2. Run sudo ./skywire-visor -c skywire-visor.json
3. Check if there is no panic