T5817: Fix for show openvpn server

In some cases we can get error:
```
Traceback (most recent call last):
  File "/usr/libexec/vyos/op_mode/show_openvpn.py", line 173, in <module>
    data = get_status(args.mode, intf)
  File "/usr/libexec/vyos/op_mode/show_openvpn.py", line 130, in get_status
    client["tunnel"] = get_vpn_tunnel_address(client['remote'], interface)
  File "/usr/libexec/vyos/op_mode/show_openvpn.py", line 66, in get_vpn_tunnel_address
    tunnel_ip = lst[0].split(',')[0]
IndexError: list index out of range
```

1 files changed, 4 insertions, 2 deletions

diff --git a/src/op_mode/show_openvpn.py b/src/op_mode/show_openvpn.py
index e29e594a5..6abafc8b6 100755
--- a/src/op_mode/show_openvpn.py
+++ b/src/op_mode/show_openvpn.py
@@ -63,9 +63,11 @@ def get_vpn_tunnel_address(peer, interface):
         # filter out subnet entries
         lst = [l for l in lst[1:] if '/' not in l.split(',')[0]]
 
-        tunnel_ip = lst[0].split(',')[0]
+        if lst:
+            tunnel_ip = lst[0].split(',')[0]
+            return tunnel_ip
 
-        return tunnel_ip
+        return 'n/a'
 
 def get_status(mode, interface):
     status_file = '/var/run/openvpn/{}.status'.format(interface)