net: tftp: Make sure timeout will not effect wrap offset

When the block 0 store to the memory of client and timeout at this
moment. Because of no ACK packet, the server will send block 0 again,
if this client reconnect to the server at this time,
TftpBlockWrapOffset will become larger than it should be.
Signed-off-by: NRockly <rocklygnome@gmail.com>
Patch: 264417

net/tftp.c

@@ -281,7 +281,7 @@ static void update_block_number(void)
 	 * number of 0 this means that there was a wrap
 	 * around of the (16 bit) counter.
 	 */
-	if (TftpBlock == 0) {
+	if (TftpBlock == 0 && TftpLastBlock != 0) {
 		TftpBlockWrap++;
 		TftpBlockWrapOffset += TftpBlkSize * TFTP_SEQUENCE_SIZE;
 		TftpTimeoutCount = 0; /* we've done well, reset thhe timeout */