When using the OpenSSL libraries in FIPS mode, the function call
EVP_DigestInit() is actually #defined to FIPS_digestinit().
Unfortunately wheres EVP_DigestInit() initialises the context and then
calls EVP_DigestInit_ex(), this function assumes that the context has
been pre-initialised and crashes when it isn't.
Bug: https://trac.libssh2.org/ticket/279Fixes#279