apps/s_server.c: Add check for OPENSSL_strdup

apps/s_server.c: Add check for OPENSSL_strdup

As the potential failure of the OPENSSL_strdup(),
it should be better to check the return value and
return error if fails.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18595)

(cherry picked from commit a6a2dd9f60b3f3e93de1337ee84f9e8f33bc86a8)