Fix a race condition 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/7523)

(cherry picked from commit 4011bab1f85d4429bad1e9388bed90a8d0da5639)