Add the check after calling OPENSSL_strdup

Add the check after calling OPENSSL_strdup

Since the potential failure of the memory allocation, the
OPENSSL_strdup() could return NULL pointer.
Therefore, it should be better to check it in order to guarantee the
success of the configuration, same as the check for
SSL_CTX_set_srp_username().

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17643)