Fix an s_server infinite loop

Commit c4666bfa changed s_server so that it asked libssl rather than the
underlying socket whether an error is retryable or not on the basis that
libssl has more information. That is true unfortunately the method used
was wrong - it only checks libssl's own internal state rather than both
libssl and the BIO. Should use SSL_get_error() instead.

This issue can cause an infinite loop because some errors could appear as
retryable when in fact they are not.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3317)