Handle the case where there is no digest in an EVP_MD_CTX

Handle the case where there is no digest in an EVP_MD_CTX

EVP_MD_CTX_ctrl assumes that a digest has always been set. However in a
non-legacy EVP_DigestSign* operation this is not the case because the
digest is handled entirely by the underlying signature implementation.

This fixes one of the travis failures on the master branch.

[extended tests]

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11317)