fuzz/asn1parse: Use BIO_s_mem() as fallback output
fuzz/asn1parse: Use BIO_s_mem() as fallback output

/dev/null is not available everywhere.

Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15672)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl