s_socket: naccept: close listening socket after accepting clients

s_socket: naccept: close listening socket after accepting clients

When `-naccept` is passed (i.e with `s_server`), the listening socket remains open while handling
client, even after `naccept` is supposed to reach `0`.

This is caused to to the decrementation of `naccept` and closing of the socket
happening a little too late in the `do_server` function.

Signed-off-by: Nadav Tasher <tashernadav@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26228)