openssl rehash: exit 0 on warnings, same as c_rehash

Fixes #6083

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6084)

(cherry picked from commit e6a833cb)