Fix error when server does not send CertificateStatus message

If a server sends the status_request extension then it may choose
to send the CertificateStatus message. However this is optional.
We were treating it as mandatory and the connection was failing.

Thanks to BoringSSL for reporting this issue.

RT#4120

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(cherry picked from commit 905943af)