If the modem is off the hook when it's stopped, hang up.

briar-core/src/net/sf/briar/plugins/modem/ModemImpl.java

@@ -91,6 +91,8 @@ class ModemImpl implements Modem, WriteHandler, SerialPortEventListener {
 	}
 
 	public void stop() throws IOException {
+		if(offHook.tryAcquire()) offHook.release();
+		else hangUp();
 		try {
 			port.closePort();
 		} catch(SerialPortException e) {