udp: make it possible to receive empty udp packets

A udp packet can have 0 content. In that case nread will be equal to 0,
but addr != NULL.

Add test case for empty data gram packets and fixed test that checked
for OOB when length == 0.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
lint: fix indentation
net: reset `errorEmitted` on reconnect

Fix for `error` events emitting only once when reconnecting
a single instance of net.Socket.

Fixes joyent/node#7888

Signed-off-by: Fedor Indutny <fedor@indutny.com>