Future proof build_chain() in x509_vfy.c

Coverity reports a potential NULL deref when "2 0 0" DANE trust-anchors
from DNS are configured via SSL_dane_tlsa_add() and X509_STORE_CTX_init()
is called with a NULL stack of untrusted certificates.

Since ssl_verify_cert_chain() always provideds a non-NULL stack of
untrusted certs, and no other code path enables DANE, the problem
can only happen in applications that use SSL_CTX_set_cert_verify_callback()
to implement their own wrappers around X509_verify_cert() passing
only the leaf certificate to the latter.

Regardless of the "improbability" of the problem, we do need to
ensure that build_chain() handles this case correctly.

Reviewed-by: Matt Caswell <matt@openssl.org>