/net/udp's headers mode gets confused by zero-length datagrams

[Former user]

/usr/inferno/emu/port/devip.c:/^ipread doesn't return any udp headers when :/^ipread/+/so_recv returns zero.

[Charles Forsyth]

changeset: 683:602fceeda569 tag: tip user: Charles Forsyth [e-mail] date: Wed Apr 01 23:47:56 2015 +0100 summary: issue 330: return headers for empty udp datagram