Properly fix possible buffer overflow

Fix for #60

Author: facchinm

src/system.c

@@ -383,8 +383,8 @@ void HAL_SPI_TxRxCpltCallback(SPI_HandleTypeDef *hspi)
   struct complete_packet *rx_pkt = (struct complete_packet *)RX_Buffer;
 
   /* Limit the amount of data copied to prevent buffer overflow. */
-  if (rx_pkt->header.size > sizeof(rx_pkt_userspace))
-    rx_pkt->header.size = sizeof(rx_pkt_userspace);
+  if (rx_pkt->header.size > SPI_DMA_BUFFER_SIZE)
+    rx_pkt->header.size = SPI_DMA_BUFFER_SIZE;
 
   /* The SPI transfer is now complete, copy to userspace memory. */
   memcpy((void *)rx_pkt_userspace, &(rx_pkt->data), rx_pkt->header.size);