Don't crash on a missing Subject in index.txt

An index.txt entry which has an empty Subject name field will cause ca
to crash. Therefore check it when we load it to make sure its not empty.

Fixes #5109

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5114)

(cherry picked from commit 1e05c6d0)