TFTP: Work around tftpd-hpa upload bug

tftpd-hpa has a bug where it will send an incorrect ack when the block
counter wraps and tftp options have been sent. Work around that by
accepting an ack for 65535 when we're expecting one for 0.

lib/tftp.c

@@ -719,7 +719,13 @@ static CURLcode tftp_tx(tftp_state_data_t *state, tftp_event_t event)
        /* Ack the packet */
        rblock = getrpacketblock(&state->rpacket);
 
-       if(rblock != state->block) {
+       if(rblock != state->block &&
+             /* There's a bug in tftpd-hpa that causes it to send us an ack for
+              * 65535 when the block number wraps to 0. So when we're expecting
+              * 0, also accept 65535. See
+              * http://syslinux.zytor.com/archives/2010-September/015253.html
+              * */
+             !(state->block == 0 && rblock == 65535)) {
           /* This isn't the expected block.  Log it and up the retry counter */
           infof(data, "Received ACK for block %d, expecting %d\n",
                 rblock, state->block);