Fix -Werror=format-truncation on GCC 7

Fix formatting

Check snprintf return for error AND overflow

src/drivers/gps/gps.cpp

@@ -684,7 +684,8 @@ GPS::task_main()
 			switch (_mode) {
 			case GPS_DRIVER_MODE_NONE:
 				_mode = GPS_DRIVER_MODE_UBX;
-				/* FALLTHROUGH */
+
+			/* FALLTHROUGH */
 
 			case GPS_DRIVER_MODE_UBX:
 				_helper = new GPSDriverUBX(_interface, &GPS::callback, this, &_report_gps_pos, _p_report_sat_info);

src/drivers/pwm_out_sim/pwm_out_sim.cpp

@@ -718,7 +718,8 @@ PWMSim::pwm_ioctl(device::file_t *filp, int cmd, unsigned long arg)
 			ret = -EINVAL;
 			break;
 		}
-		/* FALLTHROUGH */
+
+	/* FALLTHROUGH */
 
 	case PWM_SERVO_SET(0):
 	case PWM_SERVO_SET(1):
@@ -740,7 +741,8 @@ PWMSim::pwm_ioctl(device::file_t *filp, int cmd, unsigned long arg)
 			ret = -EINVAL;
 			break;
 		}
-		/* FALLTHROUGH */
+
+	/* FALLTHROUGH */
 
 	case PWM_SERVO_GET(3):
 	case PWM_SERVO_GET(2):
@@ -748,8 +750,9 @@ PWMSim::pwm_ioctl(device::file_t *filp, int cmd, unsigned long arg)
 			ret = -EINVAL;
 			break;
 		}
-		/* FALLTHROUGH */
-		
+
+	/* FALLTHROUGH */
+
 	case PWM_SERVO_GET(1):
 	case PWM_SERVO_GET(0): {
 			*(servo_position_t *)arg = 1500;

src/drivers/vmount/input_mavlink.cpp

@@ -231,7 +231,8 @@ int InputMavlinkCmdMount::update_impl(unsigned int timeout_ms, ControlData **con
 				switch ((int)vehicle_command.param7) {
 				case vehicle_command_s::VEHICLE_MOUNT_MODE_RETRACT:
 					_control_data.gimbal_shutter_retract = true;
-					/* FALLTHROUGH */
+
+				/* FALLTHROUGH */
 
 				case vehicle_command_s::VEHICLE_MOUNT_MODE_NEUTRAL:
 					_control_data.type = ControlData::Type::Neutral;