Fix OCSP_basic_verify() cert chain construction in case bs->certs is NULL

Now the certs arg is not any more neglected when building the signer cert chain.
Added case to test/recipes/80-test_ocsp.t proving fix for 3-level CA hierarchy.

See also http://rt.openssl.org/Ticket/Display.html?id=4620



Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4124)