x509_vfy.c: Fix a regression in find_issuer()

...in case the candidate issuer cert is identical to the target cert.

This is the v3.0.0 variant of #13749 fixing #13739 for v1.1.1.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/13762)