dallas: try to safeguard against bogus readings

in case wire reset() somehow allows reading to pass

code/espurna/config/types.h

@@ -406,9 +406,10 @@
 #define SENSOR_ERROR_CONFIG         11      // Configuration values were invalid
 #define SENSOR_ERROR_SUPPORT        12      // Not supported
 #define SENSOR_ERROR_NOT_FOUND      13      // Not found
+#define SENSOR_ERROR_VALUE          14      // Sensor value is invalid
 #define SENSOR_ERROR_OTHER          99      // Any other error
 
-#define SENSOR_ERROR_MAX            14
+#define SENSOR_ERROR_MAX            15
 
 //------------------------------------------------------------------------------
 // OTA Client (not related to the Web OTA support)

code/espurna/sensor.cpp

@@ -285,6 +285,9 @@ String error(unsigned char error) {
     case SENSOR_ERROR_NOT_FOUND:
         result = PSTR("Not found");
         break;
+    case SENSOR_ERROR_VALUE:
+        result = PSTR("Invalid Value");
+        break;
     case SENSOR_ERROR_OTHER:
     default:
         result = PSTR("Other / Unknown Error");

code/espurna/sensors/DallasSensor.h

@@ -318,9 +318,37 @@ private:
             return SENSOR_ERROR_CRC;
         }
 
+        if (_dataIsValid(out)) {
+            return SENSOR_ERROR_VALUE;
+        }
+
         return SENSOR_ERROR_OK;
     }
 
+    bool _dataIsValid(const Data& data) {
+        const auto all_zeroes = std::all_of(
+            data.begin(), data.end(),
+            [](uint8_t x) {
+                return x == 0;
+            });
+
+        if (all_zeroes) {
+            return true;
+        }
+
+        const auto all_255 = std::all_of(
+            data.begin(), data.end(),
+            [](uint8_t x) {
+                return x == 0xff;
+            });
+
+        if (all_255) {
+            return true;
+        }
+
+        return false;
+    }
+
     int _readScratchpad() {
         return _readScratchpad(_device, _data);
     }