MINOR: ssl: Create temp X509_STORE filled with cert chain when checking ocsp response

MINOR: ssl: Create temp X509_STORE filled with cert chain when checking ocsp response

When calling OCSP_basic_verify to check the validity of the received
OCSP response, we need to provide an untrusted certificate chain as well
as an X509_STORE holding only trusted certificates. Since the
certificate chain and the issuer certificate are all provided by the
user, we assume that they are valid and we add them all to a temporary
store. This enables to focus only on the response's validity.