Exit script if dashboard is disabled

Fixed syntax error preventing immediate exit if dashboard is disabled.

To test:
1. Flash node and run setup wizard.
2. Run `/usr/bin/commotion-bigboard-send`, then `logread`. You should NOT see this message: "user.notice luci: Dashboard config error. No listener set."
3. Add internet access and enable dashboard using "dashboard.commotionwireless.net".
4. Rerun `/usr/bin/commotion-bigboard-send`. Your node should appear in the dashboard.