net.c: Handle duplicate empty ACK
net.c: Handle duplicate empty ACK

An empty response ACK is ignored if there was a request on the Send-Q.

However if there was a subsequent duplicate ACK this would then fall through
to the general handling logic in coap_dispatch(), and would trigger the ping
handler (if set) and send a RST (if not mcast) which is not correct.

Correct a change added in by #554 so that all empty response ACKs are ignored.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap