add OpenSSL 1.1.0-pre1 compatibility

* close https://github.com/libssh2/libssh2/issues/69
* sync a declaration with the rest of similar ones
* handle EVP_MD_CTX_new() returning NULL with OpenSSL 1.1.0
* fix potential memory leak with OpenSSL 1.1.0 in
  _libssh2_*_init() functions, when EVP_MD_CTX_new() succeeds,
  but EVP_DigestInit() fails.
2 files changed