Treat an empty cookie (null string) as undefined

This allows bootstrapping a cookie file. Initially do:
  echo '' > cookie-file

In the setup script, write the received cookie value to the
cookie file, so it will be used next time the VPN is started.

Signed-off-by: Nick Andrew <nick@nick-andrew.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

main.c

@@ -179,6 +179,10 @@ int main(int argc, char **argv)
 			break;
 		case '4':
 			read_stdin(&vpninfo->cookie);
+			/* If the cookie is empty, ignore it */
+			if (! *vpninfo->cookie) {
+				vpninfo->cookie = NULL;
+			}
 			break;
 		case '5':
 			read_stdin(&vpninfo->password);