sock_udp_recv() does not fill buffer when pktsnip too big

[kb2ma]

sock_udp_recv() returns -ENOBUFS when incoming pktsnip is too big. However, it does not fill the provided buffer with as much incoming data as possible. This means the caller has no idea what was received. For gcoap, we would like to know if the incoming data is a request or a response to respond appropriately.

Submit to main RIOT issues.