test/helpers/handshake.c: Add check for OPENSSL_strdup

test/helpers/handshake.c: Add check for OPENSSL_strdup

As the potential failure of the memory allocation, the OPENSSL_strdup()
could return NULL pointer and then be assigned to 'server_ctx'.
In order to tell the callers the failure of the configuration, it should
be better to return error.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17705)