src: replace deprecated sprintf calls

src/blockchain_utilities/blockchain_stats.cpp

@@ -272,7 +272,7 @@ plot 'stats.csv' index "DATA" using (timecolumn(1,"%Y-%m-%d")):4 with lines, ''
     char buf[8];
     unsigned int i;
     for (i=0; i<24; i++) {
-      sprintf(buf, "\t%02u:00", i);
+      snprintf(buf, sizeof(buf), "\t%02u:00", i);
       std::cout << buf;
     }
   }

src/device/log.cpp

@@ -38,7 +38,7 @@ namespace hw {
   void buffer_to_str(char *to_buff,  size_t to_len, const char *buff, size_t len) {
     CHECK_AND_ASSERT_THROW_MES(to_len > (len*2), "destination buffer too short. At least" << (len*2+1) << " bytes required");
     for (size_t i=0; i<len; i++) {
-      sprintf(to_buff+2*i, "%.02x", (unsigned char)buff[i]);
+      snprintf(to_buff + 2 * i, to_len - 2 * i, "%.02x", (unsigned char)buff[i]);
     }
   }