AP_Baro: fixed a bug with corrupted conversions in MS5611 driver

the datasheet says that if you get back zero in an ADC read that the
next value can be corrupt. I have seen this happen on the FMUv1,
leading to bad altitude readings

libraries/AP_Baro/AP_Baro_MS5611.cpp

@@ -225,10 +225,21 @@ bool AP_Baro_MS56XX::_timer(void)
 
     next_cmd = next_state == 0 ? ADDR_CMD_CONVERT_TEMPERATURE
                                : ADDR_CMD_CONVERT_PRESSURE;
-    _dev->transfer(&next_cmd, 1, nullptr, 0);
+    if (!_dev->transfer(&next_cmd, 1, nullptr, 0)) {
+        return true;
+    }
 
     /* if we had a failed read we are all done */
     if (adc_val == 0) {
+        // a failed read can mean the returned value is corrupt, we
+        // must discard it
+        _discard_next = true;
+        return true;
+    }
+
+    if (_discard_next) {
+        _discard_next = false;
+        _state = next_state;
         return true;
     }
 

libraries/AP_Baro/AP_Baro_MS5611.h

@@ -51,6 +51,8 @@ protected:
     struct {
         uint16_t c1, c2, c3, c4, c5, c6;
     } _cal_reg;
+
+    bool _discard_next;
 };
 
 class AP_Baro_MS5611 : public AP_Baro_MS56XX