Fix RTCmem example buffer access past end (#4288)

Author: devyte

libraries/esp8266/examples/RTCUserMemory/RTCUserMemory.ino

@@ -38,7 +38,7 @@ void setup() {
     Serial.println("Read: ");
     printMemory();
     Serial.println();
-    uint32_t crcOfData = calculateCRC32(((uint8_t*) &rtcData) + 4, sizeof(rtcData) - 4);
+    uint32_t crcOfData = calculateCRC32( (uint8_t*) &rtcData.data[0], sizeof(rtcData.data) );
     Serial.print("CRC32 of data: ");
     Serial.println(crcOfData, HEX);
     Serial.print("CRC32 read from RTC: ");
@@ -52,11 +52,11 @@ void setup() {
   }
 
   // Generate new data set for the struct
-  for (int i = 0; i < sizeof(rtcData); i++) {
+  for (int i = 0; i < sizeof(rtcData.data); i++) {
     rtcData.data[i] = random(0, 128);
   }
   // Update CRC32 of data
-  rtcData.crc32 = calculateCRC32(((uint8_t*) &rtcData) + 4, sizeof(rtcData) - 4);
+  rtcData.crc32 = calculateCRC32( (uint8_t*) &rtcData.data[0], sizeof(rtcData.data) );
   // Write struct to RTC memory
   if (ESP.rtcUserMemoryWrite(0, (uint32_t*) &rtcData, sizeof(rtcData))) {
     Serial.println("Write: ");
@@ -90,10 +90,12 @@ uint32_t calculateCRC32(const uint8_t *data, size_t length)
   return crc;
 }
 
+//prints all rtcData, including the leading crc32
 void printMemory() {
   char buf[3];
+  uint8_t *ptr = (uint8_t *)&rtcData;
   for (int i = 0; i < sizeof(rtcData); i++) {
-    sprintf(buf, "%02X", rtcData.data[i]);
+    sprintf(buf, "%02X", ptr[i]);
     Serial.print(buf);
     if ((i + 1) % 32 == 0) {
       Serial.println();