Fix NULL dereference in SSL_check_chain() for TLS 1.3

In the tls1_check_sig_alg() helper function, we loop through the list of
"signature_algorithms_cert" values received from the client and attempt
to look up each one in turn in our internal table that maps wire
codepoint to string-form name, digest and/or signature NID, etc., in
order to compare the signature scheme from the peer's list against what
is used to sign the certificates in the certificate chain we're
checking.  Unfortunately, when the peer sends a value that we don't
support, the lookup returns NULL, but we unconditionally dereference the
lookup result for the comparison, leading to an application crash
triggerable by an unauthenticated client.

Since we will not be able to say anything about algorithms we don't
recognize, treat NULL return from lookup as "does not match".

We currently only apply the "signature_algorithm_cert" checks on TLS 1.3
connections, so previous TLS versions are unaffected.  SSL_check_chain()
is not called directly from libssl, but may be used by the application
inside a callback (e.g., client_hello or cert callback) to verify that a
candidate certificate chain will be acceptable to the client.

CVE-2020-1967

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