AP_ADSB: fixed a crash in ADSB when baro not healthy

zero pressure leads to a floating point exception

libraries/AP_ADSB/AP_ADSB.cpp

@@ -365,7 +365,9 @@ void AP_ADSB::update(void)
 
     // Altitude difference between sea level pressure and current
     // pressure (in metres)
+    if (loc.baro_is_healthy) {
         loc.baro_alt_press_diff_sea_level = baro.get_altitude_difference(SSL_AIR_PRESSURE, baro.get_pressure());
+    }
 
     update(loc);
 }