Use C99 initialisers for EVP_MD structs, for clarity, grepability and to
protect from future field reordering/removal.

No difference in generated assembly.
In tls1_cert_verify_mac(), check the return value of EVP_MD_CTX_copy_ex()
to avoid a possible NULL function call on ctx.final(). None of the callers
currently check the return value of calls to cert_verify_mac(), however
the function already returns 0 in another case and the MAC comparison will
later fail.

Issue reported by David Ramos.