AP_Baro: work around occasional 0 values from MS5611

this was seen on the PXF board. It isn't yet known why it happens

libraries/AP_Baro/AP_Baro_MS5611.cpp

@@ -363,7 +363,10 @@ void AP_Baro_MS5611::_update(void)
     _timer = tnow;
 
     if (_state == 0) {
-        _s_D2 += _serial->read_adc();// On state 0 we read temp
+        // On state 0 we read temp
+        uint32_t d2 = _serial->read_adc();
+        if (d2 != 0) {
+            _s_D2 += d2;
             _d2_count++;
             if (_d2_count == 32) {
                 // we have summed 32 values. This only happens
@@ -372,10 +375,15 @@ void AP_Baro_MS5611::_update(void)
                 _s_D2 >>= 1;
                 _d2_count = 16;
             }
+        }
         _state++;
         _serial->write(CMD_CONVERT_D1_OSR4096);      // Command to read pressure
     } else {
-        _s_D1 += _serial->read_adc();
+        uint32_t d1 = _serial->read_adc();;
+        if (d1 != 0) {
+            // occasional zero values have been seen on the PXF
+            // board. These may be SPI errors, but safest to ignore
+            _s_D1 += d1;
             _d1_count++;
             if (_d1_count == 128) {
                 // we have summed 128 values. This only happens
@@ -384,9 +392,10 @@ void AP_Baro_MS5611::_update(void)
                 _s_D1 >>= 1;
                 _d1_count = 64;
             }
-        _state++;
             // Now a new reading exists
             _updated = true;
+        }
+        _state++;
         if (_state == 5) {
             _serial->write(CMD_CONVERT_D2_OSR4096); // Command to read temperature
             _state = 0;