Add some additional NULL checks to prevent segfaults.

Add some additional NULL checks to prevent segfaults.

Fixes #14809

PR #14752 attempted to pass the libctx, propq in a few places related to
X509 signing. There were a few places that needed additional NULL checks so that they behavethe same as they did before.

OCSP_basic_sign() was changed to call EVP_DigestSignInit_ex() which passed the parameter EVP_MD_name(dgst). Since dgst can be NULL EVP_MD_name() was segfaulting.
Adding an additional NULL check EVP_MD_name() resolves this issue.

The other NULL checks are required to produce errors rather than
segfaults if the certificate is NULL.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14826)