Fix crash on empty state response

OpenVPN3 doesn't yet support "state"
management command without parameters.

While this has to be fixed on OpenVPN3
side, it doesn't mean that gui could simply crash.

Signed-off-by: Lev Stipakov <lev@openvpn.net>

openvpn.c

@@ -286,6 +286,9 @@ OnStateChange(connection_t *c, char *data)
 {
     char *pos, *state, *message;
 
+    if (data == NULL)
+        return;
+
     pos = strchr(data, ',');
     if (pos == NULL)
         return;