Rework and simplify resource flow in drbg_add

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7504)

(cherry picked from commit f9e43929c46b38667f67e02765fe0f1c0d3061d6)