examples/zcrx: fix just allocated sock struct checks

The line in process_accept() checking sockfd is a leftover from times
there was just one static socket. Now it's allocated, and we shouldn't
be checking an uninitialised value, which fails the benchmark from time
to time.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/b689b7956262aa53d37c0608c80a007dfa4cd06e.1776002658.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>