Fix the return check of OBJ_obj2txt

Fix the return check of OBJ_obj2txt

Also update OBJ_nid2obj.pod to document the possible return values.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17005)