Check return value of OBJ_nid2obj in dsa_pub_encode.

CLA: trivial

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/4600)

(cherry picked from commit 7760384b)