Sensors: Fix corner case on FD leak (not a real leak)

src/modules/sensors/sensors.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2012-2014 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2012-2015 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -1465,7 +1465,7 @@ Sensors::parameter_update_poll(bool forced)
 
 		/* this sensor is optional, abort without error */
 
-		if (fd > 0) {
+		if (fd >= 0) {
 			struct airspeed_scale airscale = {
 				_parameters.diff_pres_offset_pa,
 				1.0f,