AP_Baro: minor improvements to Baro HIL code health check

libraries/AP_Baro/AP_Baro.cpp

@@ -125,7 +125,8 @@ void AP_Baro::calibrate()
 */
 void AP_Baro::update_calibration()
 {
-    _ground_pressure.set(get_pressure());
+    float pressure = get_pressure();
+    _ground_pressure.set(pressure);
     _ground_temperature.set(get_temperature());
 }
 
@@ -166,7 +167,8 @@ float AP_Baro::get_altitude(void)
         return _altitude + _alt_offset;
     }
 
-    _altitude = get_altitude_difference(_ground_pressure, get_pressure());
+    float pressure = get_pressure();
+    _altitude = get_altitude_difference(_ground_pressure, pressure);
 
     _last_altitude_t = _last_update;
 

libraries/AP_Baro/AP_Baro_HIL.cpp

@@ -9,6 +9,7 @@ extern const AP_HAL::HAL& hal;
 // Public Methods //////////////////////////////////////////////////////////////
 bool AP_Baro_HIL::init()
 {
+    healthy = false;
     return true;
 }
 
@@ -35,11 +36,13 @@ uint8_t AP_Baro_HIL::read()
 
 void AP_Baro_HIL::setHIL(float pressure, float temperature)
 {
-    _count = 1;
-    _pressure_sum = pressure;
-    _temperature_sum = temperature;
-    _last_update = hal.scheduler->millis();
-    healthy = true;
+    if (pressure > 0) {
+        _count = 1;
+        _pressure_sum = pressure;
+        _temperature_sum = temperature;
+        _last_update = hal.scheduler->millis();
+        healthy = true;
+    }
 }