namemap: handle a NULL return when looking for a non-legacy cipher/MD
namemap: handle a NULL return when looking for a non-legacy cipher/MD

Fixes #17313

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17324)

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