Fix crash in X509_STORE_CTX_get_by_subject

If using a custom X509_LOOKUP_METHOD then calls to
X509_STORE_CTX_get_by_subject may crash due to an incorrectly initialised
X509_OBJECT being passed to the callback get_by_subject function.

Fixes #8673

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8698)