After 41891222bc landed it's possible that uv__udp_sendmsg is called
even if there are no pending write nor write completed requests:
1. User calls uv_udp_send and the request is sent immediately. The
request is the added to the completed queue and we 'feed' the uv__io
handle so that we process the completed request in the next
iteration.
2. User calls uv_udp_send again but the request is not completed
immediately, so it's queued in the write_queue.
3. The uv__io handle gets a UV__POLLOUT event and uv__udp_sendmsg is
run, which completes the send request and puts it in the
write_completed_queue. Afterwards, uv__udp_run_completed is executed
and the write_completed queue is drained.
4. At this point, the uv__io handle was made pending in step 3, in
uv__udp_sendmsg, but we no longer have requests to write or to complete,
so we skip processing.
