[lwip-members] pcb->acked bug

[Kieran Mansley]

I've just checked in a simple fix for a little bug connected to
pcb->acked.  This variable holds the amount of data that was acked by the
most recently received packet.  If, after the packet has been processed,
pcb->acked > 0 then pcb->sent (the sent callback) is called.  However, if
you receive a packet that does not acknowledge new data, pcb->acked was
not set to zero, and so pcb->sent would be called when there is nothing
for it to do.

Let me know if you can see a problem with any of this.

Thanks

Kieran